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SKIASCOPY 



AND ITS 



Practical Application to the 
Study of Refraction 



BY 



EDWARD JACKSON, A.M., M.D. 

Professor of Ophthalmology in the University of Colorado; Ophthalmologist to the Denver 

County Hospital; Consultant in Ophthalmology to St. Anthony's Hospital 

and the Mercy Hospital, Denver. 



FOURTH EDITION 



REVISED AND ENLARGED. WITH 
TWENTY-EIGHT ILLUSTRATIONS 



THE HERRICK BOOK AND STATIONERY COMPANY 
DENVER, COLORADO 

1905 



Press of 

J. B. Stott & Co. 

Denver, Colo, 





... -s* 






i\UV 




! 

1 




COPYRIGHT, 1905 
BY EDWARD JACKSON 



CONTENTS 



PREFACE 5 

CHAPTER I. 

History 7 

Name . . . . . . «• 10 

Difficulties . . . . . _ 11 

How to Study the Test . . . . . . .12 

CHAPTER II.— General Optical Principles. 

The reversal of movement . 18 

The Point of Reversal . . 20 

Real Movement of light on retina, Plane Mirror . . .21 

Real Movement of light on retina, Concave Mirror . . 23 

Apparent Movement of light in pupil ..... 25 

Rapidity of Movement of light on the retina . . „ . 27 

Magnification of the retina 29 

Form of the light area 30 

Brightness of light in the pupil 32 

Finding the point of reversal 33 

CHAPTER III.— Conditions oe Accuracy. 

The Source of light 35 

Focusing of light on the retina 37 

Position of observer for greatest accuracy 39 

Irregularities of the Media and Surfaces .... 40 

Distance of Surgeon from Patient ...... 42 

Final test . 44 

CHAPTER IV.— Regular Astigmatism. 

Two points of reversal 45 

The Band-like appearance 46 

Changes in the Light area at different distances . . .51 

Direction of movement of the band 53 

CHAPTER V.— Aberration and Irregular Astigmatism. < 

Appearance of Irregular Astigmatism 55 

Symmetrical Aberration 57 

The Visual Zone . . 58 

Appearances of Positive Aberration 59 



CONTENTS. 



Appearances of Negative Aberration 
Appearance of Conical Cornea 
Scissors-like Movement 



CHAPTER VI. — Practical Application with Plane Mirror. 

Position and Arrangement of Light 

Hyperopia 

Myopia 

Emmetropia 

^Regular Astigmatism 

Aberration and Irregular Astigmatism .... 

"Measurement of Accommodation 

"Mydriatics and Cycloplegics 



"HAPTER VII. — Practical Application with Concave Mirror. 

The Source of Light 

Hyperopia 

Myopia 

Emmetropia 

Regular Astigmatism 

Aberration and Irregular Astigmatism 

Measurement of Accommodation . 

Value of Skiascopy with Concave Mirror 



62 

64 
66 



69 
70 
72 
74 
75 
82 

84 
86 



87 
88 
89 
9i 
9i 
96 
97 
97 



CHAPTER VIII. — General Considerations. 

The Mirror 98 

The Shade 100 

Lenses and Their Support 101 

Meridian Indicators 103 

Distance Measure . . 103 

Electric Light Skiascope 104 

CHAPTER IX.— Exact Skiascopy. 

Dark Room 105 

Source of Light 106 

Modifications of Mirror 107 

The Distance 108 

Size of Pupil 108 

Pupil Stop . . 109 

Fixing Meridians . . . HO 

How to Reach Exactness 110 

Exactness Attainable .112 

-CHAPTER X.— Auto-Skiascopy. 

General Method Plane Mirror 114 

Arrangement of Light and Mirrors 115 

Concave Mirror . . 116 

What the Test will Do 117 

'INDEX . 119 



PREFACE 



A: 



.S STATED in the first edition, this book was written 
to bring about a more general adoption of Skiascopy. It 
was not then supposed that any ophthalmologist was quite 
ignorant of the test. But many who knew there was such 
a method did not know its value, or how best to use it. This 
is still too largely the case. 

The various monographs and journal articles upon 
Skiascopy, that have appeared in the last nine years, have 
done much to popularize the test ; but often in crude f orms, 
incapable of yielding very exact results. The need for em- 
phasis on its higher possibilities, and the explanation of how 
they are to be attained, is as great as ever. 

It is with this in mind that. chapter IX on Exact Skias- 
copy, has been added. And the present edition is sent out 
with the hope that what in former editions has been re- 
garded as excessive refinement and detail, will come to be 
appreciated by a larger proportion of readers. 

All experience shows that the claims made for Skias- 
copy in the first edition were moderate and well grounded. 
And the second of these especially, should receive careful 
consideration from every one concerned with the correction 
of anomalies of refraction. 

These claims are: 

First. — Skiascopy, is an objective test, independent of 
the patient's intelligence or visual acuteness, and more 
largely than any other, independent of the patient's co-op- 
eration. 

(5) 



6 PREFACE. 

Second. — It is by far the most accurate objective test. 
The limits of its accuracy depend on details of its execution, 
and the skill and patience of the observer; but, it does not 
require any rare natural qualifications, to carry it, for many 
eyes, to the extreme limits of accuracy for subjective tests. 

Third. — It requires but little more time than the use 
of the refraction ophthalmoscope or the ophthalmometer, 
which are able to give very inferior information. It saves 
time in making a complete diagnosis. 

Fourth. — It requires no costly, complex or cumber- 
some apparatus. 

Fifth. — It lays before the surgeon the refraction in 
each particular part of the pupil as it is revealed by no other 
test, opening up the principal avenue for farther advance in 
the scientific study of the refraction of the eye. 

The chapter on Auto-Skiascopy has been introduced 
both to render the treatise more complete, and in the hope 
that a certain number of students will get practical benefit 
from their acquaintance with this procedure. 

E. J. 
Denver, September, 1905. 



CHAPTER I. 

HISTORY, NAME, DIFFICULTIES, AND METHOD OF STUDY- 
ING THE TEST. 

History. — From the earliest use of the ophthalmoscope 
it has been recognized that, by the direct method, the see- 
ing of an erect image of the fundus at some little distance 
from the eye indicated hyperopia, and the seeing of an in- 
verted image indicated myopia ; and that the distance of 
the inverted image from the eye indicated the degree of 
myopia. So long ago as 1862, Bowman (Vie Royal London 
Ophthalmic Hospital Reports, Vol. II, p. 157), called atten- 
tion to the rotation of the mirror as a means of bringing 
out appearances characteristic of irregular astigmatism and 
conical cornea, and Donders, in his work on Accommodation 
and Refraction of the Eye, published in 1864, includes (p. 
490) the following note : 

" My friend Bowman recently informs me that ' he 
has been sometimes led to the discovery of regular astig- 
matism of the cornea, and the direction of the chief meridi- 
ans, by using the mirror of the ophthalmoscope much in 
the same way as for slight degrees of conical cornea. The 
observation is more easy if the optic disc is in the line of 
sight and the pupil large. The mirror is to be held at two 
feet distance, and its inclination rapidly varied, so as to 
throw the light on the eye at small angles to the perpendi- 
cular, and from opposite sides in succession, in successive 
meridians. The area of the pupil then exhibits a some- 
what linear shadow in some meridians rather than in 
others.' " 

(?) 



8 SKIASCOPY. 

The use of the ophthalmoscope above referred to, for 
the detection of irregular astigmatism, became widely pop- 
ular. It was generally adopted as the most satisfactory test 
for this kind of defect. But the observation that the same 
method was capable of revealing regular astigmatism and 
the direction of its principal meridians, does not seem to 
have attracted general attention. 

In 1872, Couper, in his paper before the Fourth Inter- 
national Ophthalmological Congress (see Trans, page 109), 
alluded to Bowman's observations, and said : "The greater 
dispersion in one meridian than in the opposite, gives rise 
to the linear shadows. Only the fact of astigmatism is 
thus established." He then went on to describe a method 
of using the ophthalmoscope as an optometer in astigma- 
tism, which is rather a modification of the ordinary use of 
the ophthalmoscope than a variety of skiascopy, since it 
depends on the recognition or non-recognition of the retinal 
vessels in different meridians, when the ophthalmoscope 
mirror is held at a considerable distance from the eye, and 
takes no account of the movement of a light area on the 
retina. 

In 1873, Cuignet, of Lille, published (Rec. d'Ophtal- 
rao/., 1873, pp. 14 and 316) an account of the test, as he 
had used it, as one capable of revealing not only the pres- 
ence of hyperopia or myopia as well as astigmatism, but 
also as giving a practical method of measuring the amount 
of these errors of refraction. He seems not to have appre- 
ciated fully the optical principles involved in the test, and 
his account of it attracted no attention. However, in 1878, 
his pupil, Mengin, introduced the practice of the method 
at Galezowski's clinic in Paris. There it was taken up, 
and Parent demonstrated its true optical basis and urged 
its advantages in a series of articles published in the Recueil 
d' Ophtalmologie in 1880-81, pp. 65 and 229. 

Lytton Forbes, in the Royal London Ophthalmic Hos~ 



HISTORY. 9 

pital Reports for 1880, (p. 62), published a paper on the test r 
giving a minute account of the various forms assumed by 
the light and shadow in the pupil, but without full expla- 
nation of their optical significance. In 1881, A. Stanford 
Morton included a full description of the test in his little 
work on the Refraction of the Eye. In 1882, Charnley gave 
the fullest demonstration of its optical basis in the Royal 
London Ophthalmic Hospital Reports, X, 3, p. 344. And Juler 
called attention to it in the Ophthalmic Review, Vol. I, p, 

3*7- 

The method described and advocated by Parent and 

those who followed him, had been that with the concave 
mirror. Cuignet had used the plane minor, and, in 1882, 
Chibret pointed out (Annates d 1 Oculistique, Vol. XXXVIII, 
p. 238) the advantages of the plane mirror in determining 
the presence and degree of myopia in the examination of 
large numbers of recruits. In 1883, Story (Ophthalmic Re- 
view, Vol. II, page 228) advocated the use of the plane 
mirror, but in the same manner as the concave, except that 
the observer should place himself at a distance of four 
metres from the patient, a distance which renders the test 
of little value for a considerable proportion of cases. 

In 1885, was published (American Journal of the Medical 
Sciences, April, p. 404) the writer's account of the test with 
the plane mirror, as applicable to all varieties of ametropia, 
the determination being made by measuring the variable 
distance of the surgeon from the patient. Since that time 
the literature of the subject has grown rapidly and the value 
of the test has been widely recognized ; but even yet it is far 
from being universally adopted and depended upon as it 
deserves to be. It must be noted that a considerable pro- 
portion of the accounts of the test bear evidence that their 
authors' acquaintance with it has been theoretical rather 
than practical, and the mass of them contribute nothing to 
the common fund of professional knowledge. The writer's 



10 SKIASCOPY. 

contributions as to the retinal illumination (Ophthalmic Re- 
view, Feb., 1890) and the relative positions of the source of 
light and the observer (Archives of Ophthalmology, July, 
1893), with the numerous suggestions of others as to the 
special pieces of apparatus to facilitate the test, which will 
be mentioned in Chapter VIII, complete the evolution of 
this method of diagnosis as now practiced. 

Name of the test. — Neither Bowman nor those who 
after him early employed the test for the detection of 
irregular astigmatism and conical cornea, proposed for it 
any special name. 

Cuignet, who brought it forward as a distinct method 
for the diagnosis of the refraction of the eye, seems to have 
thought at first that the play of light and shade in the 
pupil depended entirely on the curvature of the cornea, 
and described it under the name keratoscopie. Considering 
the real causes of the movement of light and shade in the 
pupil and the purposes for which it is employed, this name 
seems especially inappropriate. 

Parent, realizing this inappropriateness, proposed retino- 
scopie, in allusion to the fact that it was the movement of 
light and shade on the pigment layer of the retina that 
commonly gave rise to the phenomena studied. Yet this 
name was obviously open to criticism, in that the condition 
of the retina itself was not at all the matter under considera- 
tion ; and that the same play of light and shade could be 
watched on the head of the optic nerve, or, where they were 
exposed, upon the choroid or sclera. 

Chibret, to bring out the point that it was the move- 
ment of a shadow that was the subject of investigation, pro- 
posed the name of fantoscopie retinienne, and, Mr. Priestley 
Smith, probably anglicizing this term and dropping the 
allusion to the retina, called it the shadow-test. This name 
though a compound word, where a simple one should do, 
became extremely popular, and its appropriateness led Chi- 



DIFFICULTIES. 11 

bret to call to his aid, the linguistic skill of M. Egger, who 
rendered it in the term skiascopia, which in its French form 
skiascopie, or its English form, skiascopy, has been most 
widely accepted as the proper term to designate the test. 

Umhrascopy, proposed by Hartridge, is indefensible on 
linguistic grounds, and the same is true of pupilloscopie 
proposed by Landolt, and for which he aftewards offered 
the equivalent, koroscopie. Dioptroscopie was advocated by 
Galezowski (Atlas oV Ophthalmoscopie) and is appropriate, 
though equally applicable to other methods of measuring 
refraction. 

Retinophotoscopie and retinoskiascopie have been more 
recently suggested by Parent, but there seems to be no suf- 
ficient reason for retaining in the name any allusion to the 
retina. Fvndus-reflex test suggested by Oliver is also unnec- 
essarily long for a name. 

The suggestion has sometimes been made to apply one 
of these names to one form, and another name to another 
form of the test. But such a use of them is not warranted 
by the intention of their proposers or by custom. Nor is 
there any sufficient reason for the employment of separate 
names to differentiate various forms of the test ; in all its 
different forms, the test is essentially the same, the difference 
being merely as to the apparatus and mechanical detail. 

Difficulties of the Test. — That skiascopy, though a val- 
uable method of examination, is one difficult to completely 
master, becomes more and more evident as one continues 
to work with it. The theoretical basis is perfectly simple, 
the fundamental phenomena readily observed ; and, with a 
few days practice, the merest tyro may be able by it. to 
estimate the refraction in favorable eyes with an accuracy 
not to be attained by any other objective method. But 
long after the stage of such acquirement has been passed, the 
surgeon will again and again encounter cases that still prove 
difficult and puzzling. Nothing but a thorough under- 



12 SKIASCOPY. 

standing of the optical principles involved, and patient 
study of the eyes which prove most puzzling, under care- 
fully arranged favorable conditions, will enable him to 
master the test. 

The importance of the careful arrangement of the rela- 
tive positions of the light and of the observer, and the adapta- 
tions of the mirror have not heretofore been sufficiently 
insisted upon. What these adaptations and arrangements 
are will appear under their proper headings in chapters 
III and IV. It is here only necessary to emphasize their 
importance. For instance : All descriptions of the shadow- 
test allude to the characteristic band-like appearance of the 
light in astigmatism. Now, as a matter of fact, even in 
the highest degrees of astigmatism, such an appearance 
cannot be perceived, except with certain lenses, or at cer- 
tain distances in front of the eye ; and it is a distinctive 
and exact indication only when the light, the mirror, and 
the patient's and the observer's eyes are brought into a 
certain relation. 

It would be as rational to attempt to measure refraction 
with an ophthalmoscope devoid of any lens series, or to test 
the acuteness of vision in a darkened room, as to expect 
definite and satisfactory results from skiascopy, applied 
without careful attention to details that have often not 
been referred to in descriptions of the test. 

The fact that this test shows, as does no other, the actual 
refraction of the eye for each particular portion of the 
pupil, increases enormously the wealth of phenomena it 
offers for study, adding to its scientific and practical value, 
but also making it more difficult by rendering it necessary 
to discriminate between the particular portions of the 
movements of light and shade which are of practical im- 
portance^and others which are not. 

How to Study the Test. — The study of skiascopy is 
something quite different from its practical application. To 



HOW TO STUDY THE TEST. 13 

start from a few bare rules as to the placing of glasses, and 
the movements of the mirror, and the light in the pupil ; 
and attempt to learn the test by using it will never give a 
mastery of it. It is better to make a careful study of it 
before attempting to employ it as a method of ascertaining 
the refraction. 

Such a study is chiefly a use of the test, but from a 
standpoint entirely different from that of its application in 
practice. To study the test, one should as far as possible, 
start with known conditions of refraction, with lenses of 
known strength, with the eye at a known distance, and 
should observe the character of the movements of light and 
shadow in the pupil, which belong to these known condi- 
tions. In studying it, one should work from known refrac- 
tion to the pupillary appearances caused by it ; while in 
using the test for the measurement of ametropia, he has to 
deduce from observed pupillary appearances the state of re- 
fraction causing them. 

The student may, from time to time, test his progress 
towards proficiency by attempts to measure refraction by 
skiascopy, but at first familiarity with the appearances 
indicative of known conditions of refraction is chiefly to be 
sought. 

The appearances upon which the attention is fixed in 
skiascopy are those of the red reflex in the pupil. The 
first step is to learn just what the reflex in the pupil is and 
some of the variations which it may exhibit. Let the be- 
ginner, with his eye at the sight-hole of the skiascopic mir- 
ror throw into the observed eye, from a distance of 20 or 30 
inches, the light from a lamp flame, as in the ordinary oph- 
thalmoscopic examination. Looking into the observed eye 
with the light properly directed, he will see the brilliant 
point of light, the reflection from the surface of the cornea 
of the lamp flame he is using ; and he may also see reflections 
of his own face or of other objects from the surface of the 



14 SKIASCOPY. 

cornea. These are to be disregarded. The real object of 
study, the phenomena upon which attention is to be fixed, 
is the general red glow perceived within the pupil, the 
fundus reflex. 

If the mirror be rotated about an axis lying in the plane 
of the mirror, the area of light thrown by it upon the 
face will move in the direction towards which the mir- 
ror is turned. As the test becomes familiar, the direction 
of this movement will be known without any conscious 
effort to discover it. With the concave mirror at a greater 
or lesser distance than its focus or with the plane mirror at 
all distances, except at the point of reversal which it is the 
object of the test to determine, the rotation of the mirror 
also causes a movement of the red reflex in the pupil. As 
the reflex passes off across the pupil, it is followed by an 
area of shadow, and, as it returns across the pupil, the shadow 
passes out before it. The movement of the light area 
really goes on when no shadow is visible in the pupil, but 
only when light and shade are both seen can the movement 
be recognized. We know the movement of light in the 
pupil by the movement of the boundary between light and 
shade. 

Having learned what it is that he has to watch in the 
pupil, the student should make himself familiar with the 
various appearances of the fundus reflex and its movements, 
by viewing it from different distances, with different lenses 
before the eye, with different mirrors, and later in a num- 
ber of different eyes ; and all this without, at first, concern- 
ing himself especially as to the state of refraction that causes 
the particular appearance that he sees. That is, he should 
first learn to some extent what are the variations in the 
pupillary reflex. A few of them are illustrated in the fol- 
lowing pages. A second step will be the attempting to ap- 
preciate their significance. 

Without a good understanding too of the simple optical 



HOW TO STUDY THE TEST. 15 

principles underlying the test, it must remain a blind 
routine and, rule of thumb work, and can never be of the 
highest utility. To aid in such an understanding of them, 
one may, in connection with the study of the succeeding 
chapters, take a strong (15 D. to 20 D.) convex lens and a 
piece of card-board with a dot on it. The lens can repre- 
sent the dioptric media of the eye, the card-board the retina, 
and the dot the light area upon the retina. The card-board 
should be held back of the lens, a little farther than its 
focal distance, and the dot looked at through the lens from 
various distances. Nearer the lens an erect image of the 
dot (blurred of course), and, farther away, an inverted image 
will be seen, and between the two the phenomena of rever- 
sal. The movement of light on the retina may be imitated 
by a slight movement of the card in different directions. 

The apparent enlargement of the dot, as the point of 
reversal is approached, and the diminution of its apparent 
size as the point of reversal is departed from, its diffusion 
and indistinctness near the point of reversal, and its con- 
centration and greater defmiteness away from the point of 
reversal, are to be observed. Such a combination of dot and 
lens will also beautifully exhibit the phenomena of aberra- 
tion [See Chap. V] with its central and peripheral areas of 
differing movement, the one an erect and the other an invert- 
ed image. The difficulty of keeping the dot in view when 
the point of reversal is approached, will illustrate how 
small a portion of the retina is visible from the point of 
reversal when the test is applied to the eye. By holding 
in combination with the spherical lens a cylindrical lens 
of 5 D., the distortions of the light area produced by 
astigmatism, and the band-like appearances it causes at 
certain distances, should also be studied. 

This is not all to be done at a single lesson, but the 
lens and card should be kept at hand where they can be 



16 SKIASCOPY. 

used to imitate and elucidate the different conditions as 
they arise in studying the pupillary reflex. 

The study of the appearances in the eye may thus be 
carried on : Take an eye, the refraction of which is known, 
and from a distance that will give an erect movement, 
throw the light into the eye, and, by the rotation of the 
mirror, produce and study the erect movement. Then with 
a lens which it is known will give an inverted movement, 
the inverted movement is to be similarly studied. Finally 
the lens, or position of the observer is to be so varied as to 
bring the point of reversal to the observer's eye, and the 
appearance of the pupil from this point is also to be studied. 
In these studies, and, indeed, throughout the whole course, 
the student will find it easier first to master and under- 
stand the appearances with the plane mirror. 

If it is possible to get an eye free from astigmatism and 
aberration of any notable degree, the earlier studies of 
the pupillary appearances will be much simplified. After 
the appearances in such an eye have become familiar, the 
phenomena of astigmatism may be studied by placing be- 
fore the same eye, a cylindrical lens of known strength. 
The point of reversal with such a lens will give the observer 
the appearances presented by the pupil at that distance ; and 
at other distances from the eye the other appearances pre- 
sented in astigmatism can be obtained. 

For example, suppose the eye at the student's disposal 
to be hyperopic i D. Let him first place before it the convex 
2 D. lens. This will bring the point of reversal one metre 
from the eye. With the plane mirror, let him first study 
the erect movement at one-half metre ; then study the 
inverted movement at a distance of two metres ; then 
observe the eye from the point of reversal at one metre, and 
then vary his distance so as to study it from intermediate 
points. 

When he takes up the study of astigmatism, he should 



HOW TO STUDY TH£ TEST. 1 7 

place before such an eye, a convex cylindrical lens of 2 D. 
in addition to the spherical. Then from the distance of 
one-third of a metre he will be able to observe the band of 
light at right angles to the axis of the lens, from a distance 
of one metre the band of light running in the direction of 
the axis of the lens, and from other distances the other 
appearances indicative of astigmatism. 

Familiarity with the many appearances due to aberra- 
tion and irregular astigmatism will only be obtained by 
study of eyes presenting those defects. But, as the great 
majority of eyes present them in notable degree, material 
for such a study is not difficult to obtain. Careful observa- 
tion of the corresponding appearances, with the lens and 
card-board already referred to, will enable the beginner 
promptly to recognize the appearances of aberration. And, 
when once he has found an eye that presents them, let him 
carefully study them with the plane mirror, with different 
lenses, and from various distances. 

A considerable part of the study of skiascopy and espe- 
cially of the appearances of positive aberration, can be 
carried on with the aid of an artificial, schematic, or model 
eye. That of Frost is one of the best, although any, even 
the rudest, will answer. In the studies on the human eye, 
it is better to study one eye long and repeatedly, or at most 
to confine the earlier observations to a few eyes than to 
attempt to employ a large number. Each additional eye 
will introduce variations in the appearances presented, which 
will at first be only puzzling and retard, rather than assist, 
the mastery of the test. 

When the correcting' lenses chosen subjectively, differ 
from the skiascopic findings, the subjective correction (with 
the needed additional convex) should be placed before the 
eyes, and the case carefully re-studied by skiascopy. 



CHAPTER II. 

GENERAL OPTICAL PRINCIPLES. 

BRIGHTNESS OF LIGHT AREA. 

Skiascopy is a method of measuring myopia, either the 
myopia originally present in the eye or that produced by a 
lens of known strength for the purpose of measurement. 
In myopia, we have the retina situated back of the princi- 
pal focus of the dioptric media, so that rays of a certain 
divergence, that is coming from a point a certain finite dis- 
tance in front of the eye, are brought to a focus upon the 
retina. Conversely, the rays coming from a point of the 
retina and passing out through the crystalline lens and 
cornea, are brought to a focus at the same distance in front 
of the eye. The point for which the eye is focused, and 
the point on the retina, on which the focused rays are 
received, have to the refractive surfaces of the eye the rela- 
tion of conjugate foci. 

The Reversal of Movement. — The amount of myopia 
is known when we know the distance of the point in front 
of the eye, which has this relation of a focus conjugate to 
the retina. Skiascopy furnishes a method of determining 
the position of this point. Closer to the eye, than this 
point for which it is focused, the observer may see an erect 
image of the fundus. Farther from the eye than this point, 
he can perceive an inverted image. Skiascopy is a means 
of determining when the image seen is erect and when it 
is inverted, or when it passes from the erect to the inverted. 
When this occurs may be understood from a study of 
figure i, L,et M represent a myopic eye, A and B being two 

(18) 



REVERSAL OF MOVEMENT. 19 1 

points of the retina from which rays emerge to reach the ob- 
server's eye ; and C and D the points at which these rays 
coming from the retina are focused, the rays coming from 
A being focused at C and those from B at D. 

The apparent position of a point is determined by the 
direction of a ray coming from that point, and passing 
through the nodal point of the observer's eye. Suppose 
the observer's eye is placed at iV, closer than the point for 
which the observed eye is focused. . The apparent position 
of the point A is determined by a ray which passes through 
the upper part of the pupil and is turned down. It appears 



Fig. i. 

in the direction of a. The apparent position of the point 
B will be located by the ray coming through the lower 
part of the pupil and turned up. It will be seen in the 
direction of b. Thus, from this position N, the point, A 
which is really above appears above, and the point B y 
which is really below appears below. The observer sees an 
erect image. 

When, however, the observer places his eye at N\ at 
a greater distance than that for which the eye is focused, 
the ray which reaches his nodal point from A, will be 
one that comes through the lower part of the pupil and is 
turned up; so that A will appear to be located in the 
direction of o! in the lower part of the pupil. From this 
position he will judge the location of B by the ray which 
comes through the upper part of the pupil and is turned 
down, so that B will appear to be located in the direction 



20 GENERAL OPTICAL PRINCIPLES. 

of b' in the tipper part of the pupil. That is, the point A, 
which is really above, will appear to be below, and the 
point B, which is really below will appear to be above. 
The image observed is inverted. 

The Point of Reversal. — It is evident that this change 
in the relation of the rays, that brings about the change in 
the apparent position of A and B y occurs at the distance of 
the points C and D, at which, the rays coming from the 
retina are focused. Here it is that these rays intersect and 
take their new relation which gives the reversal of the 
apparent position of the points of the retina from which 
they come. 

It is, therefore, convenient in connection with skiascopy 
to designate this point as the point of reversal. The name 
indicates the significance of this point with reference to 
this test. Of course, it is really the same point as the far 
point of the myopic eye — the point for which the eye is 
focused — the conjugate focus of the retina— these latter 
names indicating the relations of the same print in other 
matters. 

It is only when the rays leave the eye convergent, only 
when the eye is myopic, that they ever come to a focus in 
front of it. If the eye be emmetropic or hyperopic, the 
rays emerging parallel or divergent remain so at all dis- 
tances. Hence, in emmetropia and hyperopia, there can be 
no point of reversal. From whatever distance the eye is 
viewed, the image perceived is erect. 

In myopia, the distance of the point of reversal from 
the eye depends on the degree of convergence of the rays 
as they leave the cornea — depends on the amount of myo- 
pia. The distance of the point of reversal from the eye, 
being the distance from the eye to its far point, is the focal 
distance of the lens required to correct the myopia. So 
that to ascertain the amount of myopia, we have only to 
determine the point of reversal, and measure its distance 
from the eye. 



THE POINT OF REVERSAL. 21 

Skiascopy determines the position of the point of re- 
versal by observation of the directions of the movement of 
light and shade in the pupil. Other kinds of ophthalmo- 
scopic examinations attempt the recognition of the details 
of the fundus image. But, as the point of reversal is ap- 
proached, the details of the fundus image become indistinct 
and fade away entirely, so that the location of the point of 
reversal cannot be accurately determined by such an exam- 
ination. On the other hand, when this point has been so 
closely approached that the fundus details are quite indis- 
tinguishable, it still remains easy to recognize the direc- 
tion of the movement of light and shade in the pupil ; and, 
from it, to deduce the erect or reversed character of the 
image. Skiascopy, therefore, determines the point of re- 
versal, and measures the degree of myopia with much 
greater exactness than Couper's or the fundus image test. 

In skiascopy, we watch the apparent movement of light 
and shade in the pupil, due to the real movement of an 
area of light upon the retina. This area of light is secured 
by reflecting into the eye the light from a lamp with a 
skiascopic mirror. This is done in a darkened room, in 
order that the retina outside of this light area may be dark, 
furnishing a decided contrast to the area to be watched. 
The movement of the light area upon the darkened retina 
is secured by varying the inclination of the mirror, rotating 
it about some axis lying in the plane of the mirror and 
passing through the sight hole. The direction of the move- 
ments thus produced by a certain change in the inclination 
of the mirror depends on whether it is plane or concave. 

Real Movement of the Light on the Retina. The 
Source of Light. — The lamp flame, or similar source of 
light used for the test, may be called the original source of 
light, in contra-distinction to the reflection of it from the 
mirror, which being more immediately related to the move- 
ment of the light on the retina, we shall call the immediate 
source of light. 



22 GENERAL OPTICAL PRINCIPLES. 

The Plane Mirror. — With the plane mirror the imme- 
diate source of light is behind the mirror as far as the 
original source of light is in front of it. The rays reflected 
from the mirror enter the eye under observation as though 
they had started from this immediate source. As the 
mirror is rotated, the apparent position of the immediate 
source of light changes ; for this immediate source is sit- 
uated upon a line drawn through the original source per- 
pendicular to the surface of the mirror, and necessarily 
changes with that perpendicular as the inclination of the 
mirror changes. 

With the change of position of the immediate source 
of light, the rays coming from it and falling upon the eye, 
are made to fall upon a new part of retina, and thus the 
inclination of the mirror causes a change in the part of 
the retina that is lit up by the light reflected into the eye. 




i 1 b 



Fig. 2. 

What these changes are can be better understood by a 
study of figure 2. L represents the position of the lamp 
flame, the original source of light. When the mirror is 
held in the position .4.4, the immediate source of light is 
situated at /, and light entering the eye from that direction 
falls upon the retina toward a. When, however, the posi- 
tion of the mirror is changed to BB, the immediate source 
of light is changed to /', from which, light falls upon the 
retina toward b. As the mirror is rotated from .4.4 to BB, 
the position of the immediate source of light moves from I 
to V ) and, as a consequence, the area of light upon the 



MOVEMENT OF LIGHT ON THE RETINA. 23 

retina moves from a to b. The light on the retina then, 
moves in the direction that the mirror is made to face. It 
is said to move with the mirror. 

Only a portion of the light reflected by the mirror 
enters the eye, the remainder falls upon the face and makes 
an area of light on the face. One may readily demonstrate 
by trial that this area of light cast by the mirror on the 
face also moves ivith the mirror under all circumstances. 

The rays of light coming from I and V intersect at the 
nodal point of the eye ; and passing directly on do not again 
change their relative positions. Whatever the distance of 
the retina from this nodal point, the movement of the light 
upon it will be in the same direction, so that whether the 
retina be at H. as in hyperopia, at E. as in emmetropia, or 
at M. as in myopia, the real movement of light upon it from 
a certain movement of the mirror is always in the same 
direction. 

Therefore, with the plane mirror, the real move- 
ment of the area of light on the retina is with the mirror — 
with the area of light on the face — in all states of refraction. This 
is true for all distances of the light from the mirror, or of 
the light and mirror from the tested eye. 

The Concave Mirror. — With the concave mirror as 
used in skiascopy, the immediate source of light is a real 
focus of the mirror, conjugate to the position of the light, 
situated between the mirror and the eye to be tested. The 
position of this immediate source varies with the position 
of the mirror, moving in the direction that the mirror is 
made to face and causing an opposite movement in the 
area of light that falls from it upon the retina. 

In figure 3 L again represents the original source of 
light. When the mirror is in the position AA^ the light 
falling upon it from L is focused at l, and the little inverted 
image of the lamp flame there formed is the immediate 
source of light. From it the rays diverge, some to fall 



24 



GENERAL OPTICAL PRINCIPLES. 



upon the face, and those entering the eye to fall upon the 
retina toward a. When the mirror is turned to occupy the 
position BB, the light falling upon it is focused at /', which 




Fig. 3. 

becomes the new position of the immediate source of light,, 
and from which the rays entering the eye fall upon the 
retina toward b. As the mirror is rotated from A A to BB, 
the immediate source of light moves from I to V and the 
light upon the retina from a to b. This will be the direc- 
tion of its movement in all states of refraction whether the 
retina be situated at H. as in hyperopia, at B. as in emrne- 
tropia, or at M. as in myopia. The portion of the light 
which falls upon the face, however, and forms the facial 
area, as can be readily demonstrated by trial, moves in the 
direction that the mirror is made to face. 

We have then : With the concave mirror, the real 
movement of the area of light on the retina is against the 
mirror, and against the light on the face, in all states of refrac- 
tion. And when, in the following pages, ''erect " or ''direct " move- 
ment is spoken of with the concave mirror, such movement 
" against " the mirror is meant. 

The above is the movement that occurs with the con- 
cave mirror used as in skiascopy so far from the original 
source of light and from the eye to be tested, that the con- 
jugate focus of the original source of light falls in front of 
the eye. If, however, the original source of light be brought 
so close to the mirror that the rays from it are not rendered 
convergent, but continue to diverge after reflection, the 



APPARENT MOVEMENT IN THE PUPIL. 25 

immediate source of light will be a magnified image of the 
lamp flame, situated behind the mirror as in the case of the 
plane mirror ; and the movement of the retinal light area 
will be precisely the same as with the plane mirror. Again, 
if the rays reflected by the mirror are rendered convergent, 
but the eye to be tested is brought so near that they cannot 
come to a focus in front of its nodal point, the light will 
pass in as though from an immediate source back of the 
mirror, and the movement of the area of light on the retina 
will again be like that with the plane mirror. If the light 
reflected upon the eye be convergent so as to be focused 
just at its nodal point, no movement of light on the retina, 
such as we have been considering, will occur ; but whatever 
direction the mirror is turned, so long as the light enters 
the eye, the retinal light area will remain stationary. 

It is to be borne clearly in mind that the, movement so 
far spoken of is the real movement of the light area upon 
the retina,- as it would appear from within the eye itself, or 
when viewed from behind the retina, with the sclera and 
choroid cut away. 

The Apparent Movement of the Light in the Pupil. 
— What we observe in skiascopy, however, is the apparent 
movement of the light in the pupil as viewed from the 
position of the observer some distance in front of the eye. 
When an erect image of the retina is viewed, this apparent 
movement of the light will be in the same direction as the 
real movement. When an inverted image is viewed, the appar- 
ent movement will be in the direction opposite to that of the 
real movement. 

The observer can always watch the movement of the 
light area on the face, and know that with the plane mir- 
ror the light area on the retina always has a real movement 
in the same direction, and with the concave mirror it always 
has a real movement in the opposite direction ; and he has 
only to compare the apparent movement of the light which 



26 GENERAL OPTICAL PRINCIPLES. 

he watches in the pupil with the known direction of the 
real movement on the retina, to determine whether he sees 
an erect or an inverted image. When the apparent and 
real movements are in the same direction, he knows (page 
1 8) he is looking at the eye from a distance shorter than 
that for which it is focused. When the apparent and real 
movements are in the opposite directions, he knows that he 
is looking at the eye from a distance greater than that for 
wmich it is focused. 

The direction of the apparent movement of the light 
then, will be with the light on the face in hyperopia and 
in emmetropia at all distances, and in myopia when the 
eye is viewed from a point nearer than its point of reversal. 
The apparent movement in the pupil will be the opposite 
of the real movement only in cases of myopia when the eye 
is viewed from somewhere beyond its point of reversal. 

With the plane mirror, the apparent movement is with 
the light on the face in hyperopia, emmetropia, and myopia with 
the point of reversal behind the observer, and against the light 
on the face in myopia viewed from beyond the point of reversal. 
With the concave mirror the apparent movement is 
against the light on the face in hyperopia, emmetropia, and 
myopia with the point of reversal behind the observer ; and is 
with the light on the face only in myopia viewed from beyond 
the point of reversal. This statement made to conform to 
the practice customary in the use of the concave mirror 
[where the observer keeps a constant distance of i metre 
from the eye, corresponding to i D. of myopia] , would be : 
the light moves against the light on the face and against the 
mirror in hyperopia, emmetropia, and myopia of less than 1 D., 
and only moves with the light on the face in myopia of more 
than 1 D. 

When we speak of " inverted " movement with the 
concave mirror, this movement with the mirror and with 
the light on the face is meant. 



RAPIDITY OF MOVEMENT. 27 

These statements have been made with reference to 
the apparent movement of the light before the state of re- 
fraction has been modified by any glass placed before the 
eye for that purpose. But they hold equally as to hyper- 
opia, emmetropia, or myopia remaining uncorrected or pro- 
duced by a lens placed before the eye. For instance : — In 
myopia the movement remains against the light on the face 
with the plane mirror, or with the light on the face with 
the concave mirror, so long as the concave lens employed 
is not strong enough to bring the point of reversal to the 
distance of the observer's eye. In hyperopia or emmetro- 
pia, where the movement is watched through a convex 
lens, the movement remains with the light on the face for 
the plane mirror, and against the light on the face for the 
concave mirror, until a convex lens is used, that is strong 
enough to over-correct the hyperopia and cause enough 
myopia to bring the point of reversal nearer to the eye 
than the position of the observer. 

Rapidity of Movement of the Light on the Retina. 
— The rapidity with which the light and shadow appear to 
move across the pupil depends first, on the rapidity of the 
real movement of the light area upon the retina ; and, 
second, upon the magnification of the retina. The rapidity 
of the real movement on the retina depends : 

On the rate of movement of the mirror in the observ- 
er's hand. 

On the distance of the mirror from the observed eye. 

On the distance of the original source of light from 
the mirror. 

And upon the distance of the retina from the nodal 
point in the observed eye. 

The rate of movement of the mirror and the distance 
of the light from the mirror determine the rapidity of the 
movement of the immediate source of light ; this being 
greater as the mirror is moved more quickly, or as the ori- 



28 GENERAL OPTICAL PRINCIPLES. 

ginal source of light is more distant from the mirror. The 
excursion which the immediate source of light can make 
is limited by the width of the mirror, and the extent of 
movement of the light area on the retina produced by the 
movement of the immediate source of light entirely across 
the mirror depends on the relative distance of the mirror 
and the retina from the nodal point of the eye. The wider 
the mirror, or, the nearer it is to the nodal point of the 
eye, or the farther the retina is from that nodal point, the 
greater the extent of movement produced in the retinal 
area of light by a given movement of the mirror. On ac- 
count of the relative distances of the retina from the nodal 
point, the extent of the movement of the light on the retina 
is, other things being equal, least in the highest hyperopia 
and greatest in the highest myopia. 

The rapidity of the real movement of the light on the 
retina then, is increased : 

By moving the mirror faster. 

By carrying the original source of light farther from 
the mirror. 

By bringing the mirror closer to the eye. 

By elongation of the antero-posterior axis of the eye- 
ball. 

The real movement of the light upon the retina is 
made slower: 

By moving the mirror more slowly. 

By bringing the original source of light closer to the 
mirror. 

By carrying the mirror farther from the eye. 

By shortening of the antero-posterior axis of the eye- 
ball. 

In using the test, the distance of the light from the 
mirror is, for most purposes, practically constant, and the 
ordinary variations in the antero-posterior axis of the eye- 
ball are so slight as to have no appreciable influence. So 



MAGNIFICATION OF THE RETINA. 29 

that the rapidity of the real movement of light on the 
retina depends principally on the rapidity of the movement 
of the mirror and the distance of the mirror from the eye. 

Magnification of the Retina. — In practice the rapidity 
of the apparent movement of the light in the pupil depends 
far more on the extent to which the retina, and the real 
movement of light upon it are magnified, than upon the 
actual rate of that real movement. The retina, as viewed 
through the pupil from different distances, is seen under 
different degrees of magnification. When the observer's 
eye is placed at the point of reversal, the rays from a single 
point of the retina, passing through all parts of the pupil, 
converge to the observer's nodal point, so that the one point 
of the retina appears to occupy the whole of the pupil, and 
the retina is seen indefinitely magnified. As the observer's 
eye departs from the point of reversal, it receives the rays 
from an increasing area of the retina, more and more of the 
retinal image occupies the same space of the pupil and the 
retina is seen less magnified. 

This is illustrated in figure 4, which represents an eye 
with its point of reversal at A. If the observer's eye be 
placed at A it receives rays only from the point a, and this 
point appears to occupy the whole pupil. If, however, the 
observer's eye be placed at B, from which rays would be 



fig. 4. 



focused at b behind the retina, and, at which, rays from b 
would be focused, the observer will be able to see in the 
space of the pupil all of the retina, m n included, between 
the broken lines passing from B to b — all of the retina, 



30 GENERAL OPTICAL PRINCIPLES. 

which would receive a circle of diffusion if the rays were 
coming from the point B. Or, again, if the observer's eye 
be placed at C, from which rays will be focused at c in front 
of the retina, and, at which, rays coming from c would be 
focused, he will be able to perceive the portion of the re- 
tina, m n included, between the dotted lines, passing 
through c and continued on to the retina — the area upon 
which would be formed a circle of diffusion by rays coming 
from the point C. 

It follows then that the closer the observer's eye to the 
point of reversal, the more is the real movement of light upon the 
retina magnified, and, therefore, the swifter does it appear. The 
farther the observer's eye is removed from the point of reversal 
the less is that real movement of light on the retina magnified ; 
and the shiver is the apparent movement as watched in the pupil. 

And, as this source of variation overcomes all other 
sources of variation in the rate of the apparent movement 
of the light [except the rate of rotation of the mirror, which 
is, to a considerable extent, under the control of the ob- 
server] , the rapidity of the apparent movement of light and shade 
in the pupil increases as the point of reversal is approached, and 
diminishes as that point is departed from, and constitutes a 
measure of the degree of ametropia remaining uncorrected. 

Form of the Light Area. — The real form of the light 
area on the retina, except under certain conditions in astig- 
matic eyes, will be circular. If the light be perfectly 
focused on the retina it is circular, because that is the form 
of the source of light employed (see Chapter III). If the 
light be not perfectly focused on the retina, the circular 
pupil gives its form to the resulting area of diffusion. 

The influence of regular astigmatism on the apparent 
form of the light area as seen in the pupil will be discussed 
in Chapter IV ; and the influences of irregular astigmatism 
and aberration in Chapter V. These influences, especially 
the latter, are really dominant, and of the greatest practical 



FORM OF THE LIGHT AREA, 



31 



importance in nearly all eyes. But in eyes free from such 
defects the form varies with the departure of the observer's 
eye from the point of reversal. If the magnification of the 





Fig. 5. 



Fig. 6. 



retina is so slight that all of it occupied by the light area 
is visible in the pupil at one time, that area appears circu- 
lar, as represented in figure 5. But when the point of re- 
versal is approached so that the magnification of the retina 
prevents all of the retinal light area from being seen at one 
time, only a portion of its outline is visible as an arc of the 
greatly enlarged circle, as shown in figure 6 ; and the nearer 
to the point of reversal that the observer comes, the nearer 
does the boundary between light and shade approach to a 
straight line. It must be borne in mind, however, that this 
is still part of the boundary of a circle, and hence that dif- 
ferent parts will run in all the different directions , in con- 
tradistinction to the band-like appearance of astigmatism, 
the direction of which always conforms to one or the other 
of the principal meridians. 

From the point of reversal, however, but a single 
point of the retinal light area could be visible to the ob- 
server at a time, so that the form of that area could not 
from this position influence the form of light and shade 
apparent in the patient's pupil. From this single luminous 
point of the patient's retina the light is not focused on the 
observer's retina, but falls there in an area of diffusion 
which would take its form from the pupil of the observer, or 



32 GENERAL OPTICAL PRINCIPLES. 

rather from the sight-hole of the mirror, which is the smaller. 
This being circular would give a circle of diffusion, the por- 
tion of which was referred to the patient's pupil, giving its 
shape to the apparent light area. In practice, however, we 
do not produce just these conditions. For reasons referred 
to under the next heading we do not commonly watch the 
movement of light and shade from exactly the point of re- 
versal. Still, as this point of reversal is approached, this 
form of the diffusion area on the observer's retina often 
exerts some influence on the apparent form of light and 
shade in the patient's pupil. This may be demonstrated by 
trial with a square or triangular sight-hole. For a full 
discussion of this matter the reader is referred to a paper 
by Dr. Carl Weiland in the Medical News, October 12th, 
1895; and one by the author Annals of Ophthalmology and 
Otology, April, 1896. 

Brightness of the Light in the Pupil. — This depends 
on the illumination of the retinal light area and the extent 
to which that area is magnified. 

The illumination of the light area on the retina de- 
pends on the brightness of the original source of light and 
the accuracy with which the light coming from it is focused 
on the retina. The brighter the source of light and the 
more accurately it is focused, the brighter the illumination 
of the retina. The dimmer the light and the larger the 
circle of diffusion over which it is dispersed, the more feeble 
the retinal illumination. 

As the immediate source of light is usually near the 
mirror (in front for the concave, behind for the plane) when 
the mirror and the observer's eye approach the point of re- 
versal, or the point of reversal is brought to them by a 
change of lenses, the light is more nearly focused on the 
retina, and the actual illumination of the light area in the 
patient's eye brighter. 

But, as the point of reversal is approached, the appar- 



BRIGHTNESS OF THE LIGHT IN THE PUPIL. 33 

ent brightness of the light area in the pupil is diminished by 
the increasing magnification of the retina, which causes the 
light from a smaller part of the retinal area to occupy the 
whole space of the pupil. Again, when the observer is near 
the point of reversal, the part of the retina that he can see 
is the part to which light can be reflected only from the 
immediate vicinity of the sight-hole and the sight-hole 
itself. On this account the illumination of this part of the 
light area is feeble as compared with the illumination of 
other parts of the light area ; and if the refraction through- 
out the pupil were uniform there would be a central portion 
in complete darkness. This is, however, prevented by the 
astigmatism and aberration, present in all eyes, which pre- 
vent the perfect focussing of the light even when the 
immediate source of light is exactly at the point of re- 
versal. 

On this account the brightest apparent illumination of 
the pupil is never obtained at the point of reversal, but 
usually at one or two dioptres from it, the exact position 
being dependent on the arrangement of the source of light. 
Finding the Point of Reversal. — The point of reversal 
is to be recognized only when the observer's eye is in its 
immediate neighborhood. This may be effected either by 
varying the distance of the observer's eye from the observed 
eye until it comes to the position of the point of reversal, 
or by varying the position of the point of reversal by 
changes in the lenses placed before the observed eye until 
the point of reversal comes to the chosen position of the 
observer's eye. For reasons to be stated in Chapter VI, the 
former method is the better when using the plane mirror, 
and the latter is to be resorted to when the concave mirror 
is employed. In any case, the trial movement across the 
pupil shows by the direction of the movement whether a 
point of reversal exists between the observer and the ob- 
served eye, and the rapidity of movement shows approxi- 



34 GENERAL OPTICAL PRINCIPLES. 

mately [when the observer has learned to appreciate its 
significance] the extent of the interval between the position 
of the observer and the point of reversal. If the movement 
be slow, the interval is large, perhaps several dioptres. If 
it be rapid, the interval is less. 

Upon these data of the direction and rapidity of the 
movement, the surgeon bases the next step of the test, the 
selection and placing of lenses before the eye. This beings 
done, the test is repeated, the movement seen through 
the lens noted, both as to its direction and rapidity, 
and the distance of the observer from the patient, or the 
strength of the lens before the observed eye, varied in 
accordance therewith. This process is continued until the 
observer's eye reaches the point of reversal, or the point of 
reversal is brought by the lens to the observer's eye. But 
the test should not be regarded as complete until the movement has 
been repeatedly viewed both from within and from beyond the 
point, of reversal, as well as from that point. Only by this 
precaution of observing from a slightly greater and a 
slightly less distance, or with a slightly stronger or slightly 
weaker lens than that which brings the point of reversal to 
the surgeon's eye, can the certainty of a correct result be 
assured. 



CHAPTER III. 

CONDITIONS OF ACCURACY, 

Since in skiascopy one has to observe the movement 
of an area of light across the shaded retina, the size, bright- 
ness and sharpness of the contrast between the margin of 
this light area, and the shadow immediately adjoining it 
are very important factors in determining the definiteness 
and accuracy of the test. For reasons discussed in the pre- 
ceding chapter, the contrast between light and shadow as 
seen in the pupil, necessarily diminishes as the point of 
reversal is approached. It is, therefore, important to have 
the contrast between light and shadow upon the retina as 
sharp as possible. 

Darkening the Room.— To- secure this contrast, the 
retina outside of the proper light area should be in absolute 
darkness. This requires a complete darkening of the room 
in which skiascopy is practiced, including the shading of 
the source of light, except in the direction in which it is 
used. The difference in the ease of the test applied in a 
completely darkened room, can only be appreciated by one 
accustomed to applying it under the former condition. 

The Source of Light. — To secure the brilliant illumi- 
nation of the light area, in contrast with the complete 
shadow around it, the source of light must be as bright as 
possible. On account of the difficulty about the sight hole 
to be referred to later, the arc electric light cannot be 
employed, except to illuminate a piece of ground glass as 
suggested by Derby. The incandescent electric light is 
(except in the special form referred to on page 104) 

(35) 



36 CONDITIONS OF ACCURACY. 

not available on account of its form, so that recourse must 
be had to one of the various illuminating flames. Of these 
the acetylene flame is most brilliant. Xext come paraffin, 
the heavy mineral oils, and gas flames reinforced with the 
richer hydro-carbons, or used on the YVelsbach mantle, and 
after this the ordinary illuminating gas. But a good flame 
of the latter furnishes a satisfactory illumination. 

It is more important, whatever name is used, that the 
"brightest part of it should be employed. With all flames 
there is at the margin a comparatively gradual shading 
from light to darkness, which interferes with the sharpness 
of the boundary of the light area on the retina. To secure 
that sharp boundary as well as to prevent the diffuse illu- 
mination of the room, and to limit the size of the source of 
light, the flame should be entirely covered by an opaque 
shade with an aperture of the proper size placed opposite 
the most brilliant part of the flame. This gives, under 
proper conditions of focusing, a perfectly sharp margin to 
the light area on the retina. 

The size of this opening in the opaque screen deter- 
mining the size of the original source of light is governed 
by various conflicting requirements. The smaller the 
source of light, the more characteristic its shape in regular 
astigmatism, and the easier to distinguish the diriere:;: 
movements in different parts of the pupil in irregular astig- 
matism and aberration. But enough light must be furn- 
ished by it to give a distinct area of light upon the face, as 
well as to give sufficient illumination within the pupil ; and 
the source of light must be considerably larger than the sight 
hole in the mirror. As the mirror is rotated, the imme- 
diate source of light appears to move across it, and if this 
source were not larger than the sight hole, it would, at 
times, entirely disappear within that opening. At such 
times, no light would fall upon the retina and the illumina- 
tion would disappear entirely from the pupil, causing delay 



FOCUSING OF THE LIGHT ON THE RETINA. 37 

and uncertainty in the test. If, however, the immediate 
source of light is larger than the sight hole, no such dis- 
appearance of the light occurs. 

The size of the opening through which light is ob- 
tained, is then a compromise between the requirements of 
light and the size of the sight hole on the one hand, and 
a need to have the retinal light area as small as possible on 
the other. The diameter of the source of light for accurate 
work with the plane mirror may generally be reduced to 
little more than twice the diameter of the sight hole. In 
practice the writer prefers an aperture five millimetres in 
diameter, but the beginner may find one of double that 
diameter more satisfactory. 

The Sight Hole. — This must be large enough to allow 
the observer to readily watch through it the movement of 
light on the face, as well as in the pupil. This will be 
facilitated by the thorough darkening of the room, and the 
complete freedom of the sight hole from reflections. With 
this limitation, the sight hole should be as small as possi- 
ble, to reduce the area from which little or no light is 
reflected into the eye, and to reduce the circles of diffusion 
formed on the observer's retina when he watches the move- 
ment of light and shade from the immediate vicinity of the 
point of reversal. A sight hole two millimetres in diame- 
ter has rendered the writer the best service. 

Focusing of the Light on the Retina. — When the rays 
coming from the immediate source of light are accurately 
focused upon the retina, the area of retinal illumination 
will be the smallest and brightest, and will have the most 
definite edge. This accurate focusing is secured only when 
the immediate source of light is situated at the focus 
conjugate to the retina — the point of reversal. In search- 
ing for the point of reversal, it is, therefore, advantageous 
to keep the immediate source of light as close to the 
observer's eye and the mirror as possible. 



38 CONDITIONS OF ACCURACY. 

With the plane mirror the immediate source of light 
is a reflection of the original source as far behind the 
mirror as the immediate source is in front of it. The 
closer the original source of light can be brought to the 
mirror, the closer will its reflection be to the observer's eye ; 
and to the point of reversal, at the critical moment when 
the observer's eye reaches that point. The original source 
of light then should be kept as close to the mirror as pos- 
sible. On this account it should be moveable, to follow the 
movements of the observer's eye and the mirror, when the 
distance of these from the eye under observation is varied. 

When the observer withdraws to the distance of two 
metres or more from the patient, it may not be practicable 
to keep the light very close to the mirror, but at such a 
distance, the separation of the source of light from the 
mirror becomes of small importance. ' For, if the original 
and immediate sources of light were at the mirror, the rays 
from the latter would have a divergence of one-half dioptre 
when they reached the eye ; and, if the original source of 
light were one metre in front of the mirror, so that the 
immediate source would be one metre behind the mirror, 
that is three metres from the eye, the rays from it would 
reach the eye one-third dioptre divergent, and the difference 
between the one-half and the one-third dioptre of divergence 
is so trifling as to be in this connection of no practical 
importance. 

On the other hand, when the surgeon approaches close 
to the patient's face, the slight distance that must necessa- 
rily remain between the original source of light and the 
mirror becomes a source of imperfect focusing of the light 
on the retina, and therefore of inexactness in the deter- 
mination of the point of reversal. Suppose the mirror to 
be at five inches from the eye and the original source of 
light three inches from it, this will make the immediate 
source of light eight inches from the eye, and the rays from 



POSITION OF THE OBSERVER. 39 

it will reach the pupil 5 D. divergent when the surgeon is 
seeking the point of reversal corresponding to 8 D. of myo- 
pia. This difference of 3 D. interferes greatly with the 
delicacy of the test. 

With the concave mirror, the immediate source of light 
being a real image of the original source in front of the 
mirror, at its focus conjugate to the position of the original 
source, cannot be brought closer to the mirror than its 
principal focal distance. It is brought closest by carrying 
the original source of light as far away from the mirror as 
possible. The original source of light then, for the con- 
cave mirror, should be behind the patient as far as possible. 
The important exception to these rules for placing the 
light, necessary for the most accurate determination of the 
principal meridians in regular astigmation is discussed in 
connection with that subject in Chapter IV. 

Position of Observer for Greatest Accuracy.— With 
the plane mirror the immediate source of light is necessarily 
behind the mirror. It will, therefore, be exactly at the 
point of reversal when the mirror and the observer's e> e 
are slightly within the point of reversal. Hence the condi- 
tions of accuracy are better complied with for the observa- 
tion that is made from within the point of reversal, where 
the light still moves in the pupil with the light on the 
face, than for the observation that is made from beyond the 
point of reversal, where movement is inverted. The point 
of reversal is then, with the plane mirror, most closely 
approximated from the side toward the observed eye ; and 
in practice the greatest accuracy is attained by considering 
that the point of reversal is located at the greatest distance from 
the eye at ivhich erect movement can be seen in the visual zone of 
the pupil. 

With the concave mirror the immediate source of light 
being necessarily in front of the mirror, can be brought 
accurately to the point of reversal only when that point of 



40 CONDITIONS OF ACCURACY. 

reversal is the focal distance of trie mirror in front of the 
observer's eye. The point of reversal then is approached 
more accurately with the lens which still leaves it in front 
of the observer's eye, than with the lens which removes it 
back of the observer's eye. Hence, with the concave mirror, 
the strongest concave lens, or the weakest convex, which allows the 
movement of the light iii the pupil with the light on the face, is 
the Ins which brings the point of reversal mod accurately to the 
distance chosen. 

In regular astigmatism, as will be shown in chapter 
IV, the position of the light must be specially arranged, 
when it is desired to develop the band-like appearance 
characteristic of that condition. For the measurement of 
refraction in either of the principal meridians, the adjust- 
ment should be precisely the same as for simple hyperopia 
or myopia. But the band-like appearance cannot certainly 
be recognized unless the necessary conditions there ex- 
plained, as to the position of the observer and source of 
light, are carefully observed. When the proper precautions 
are taken one can get a characteristic band of light with 
even one-half dioptre of astigmatism, and by that band can 
fix the direction of the principal meridians with great 
accuracy. 

In the higher degrees of regular astigmatism there is 
considerable difficulty in measuring the refraction of the 
principal meridians with accuracy. It is, therefore best, 
before regarding the skiascopic test as completed, to place 
before the eye such a cylindrical lens as appears to be 
required to correct the astigmatism, repeat the test, and so 
ascertain whether the astigmatism has been accurately cor- 
rected. Fuller references to this matter will be found in 
the Chapters VI and VII. 

Irregularities of the Media and Surfaces. — These in- 
terfere with skiascopy not only by changes in the apparent 
movement of the light as watched in the pupil, but also by 



IRREGULARITIES OF THE MEDIA AND SURFACES. 41 

preventing the perfect focusing of the light which falls 
upon the retina, and in this way, they limit to some extent, 
the accuracy of the test, since they are present in some 
degree in nearly all eyes. 

In the case of positive aberration (see Chapter V), the 
interference with focusing is of the same kind as the defect 
in the refraction of a strong convex spherical lens. If one 
takes such a lens and intercepts with a piece of card-board 
the narrowing pencil of rays that have passed through the 
lens, he will find that the strong refraction at the margin 
of the lens causes a ring of condensation at the periphery 
of the circle of diffusion. This ring is exhibited from 
close behind the lens back to its principal focus, beyond 
which, we have the condensation at the centre of the light 
area and a gradual fading away of light around it. Hence, 
the circle of diffusion in front of the principal focus pre- 
sents a brilliantly illuminated edge in sharp contrast with 
the shadow around it, while at the principal focus and be- 
hind it, the light area has not a sharply defined edge, but 
fades gradually into the shadow around it. 

Therefore, in making the test, the influence of posi- 
tive aberration upon the distribution of light in the light 
area may be utilized by having the light focused not exactly 
on the retina, but slightly back of it. This may be brought 
about by having the immediate source of light closer to 
the observed eye than are the observer's eye or the point 
of reversal ; conditions that are secured in the use of the 
concave mirror. Hence, for positive aberration of a certain 
distribution in the pupil, a more definitely bounded light 
area might be obtained by the use of the concave mirror 
than could be had with the plane mirror. But the dis- 
advantage of an indistinct margin of the retinal light area 
due to aberration is partly or entirely balanced by the 
obliteration from that area by this same aberration of the 
unilluminated or poorly illuminated centre due to the 
sight-hole. 



42 CONDITIONS OF ACCURACY. 

With negative aberration, where the refraction is 
weaker near the periphery of the pupil, the condensation 
ring of light is less pronounced and is found back of the 
principal focus for the central visual area. For this form 
of aberration the plane mirror, enabling the observer by 
pushing the source of light from the mirror to get the light 
focused in front of the retina, has some advantage over the 
concave mirror. 

The interference with the focusing of the light on the 
retina due to irregular astigmatism cannot be overcome in 
any way, and it impairs the value of the test and makes it 
more difficult to apply in eyes presenting marked defects 
of this kind. 

Distance of the Surgeon from the Patient. — It will 
always be impossible to determine the point of reversal 
with perfect exactness. The best that can be done is to 
make out that it lies, within narrow limits of possible 
error, at about a certain distance. It may be a little nearer, 
it may be a little farther off. 

If the distance be a short one, if the lens used be such 
that the point of reversal is brought close to the observed 
eye, the probable inaccuracy of distance will cause an 
appreciable error in estimating the refraction, measured in 
dioptres. For instance : at eight inches from the eye, two 
inches additional, making ten inches, will correspond to a 
whole dioptre of refraction, and two inches less, making 
the distance six inches, will correspond to a difference of a 
dioptre and a half. On the other hand, at eighty inches, 
a foot either way will correspond to less than one-quarter 
of a dioptre of inexactness. Hence, for accurate work it 
is best to make the determination of the point of reversal 
at the greatest distance at which it can be certainly made 
in the visual zone. The importance of this has been espe- 
cially dwelt upon by Randall (Trans. Section on Opthahnol. 
A in. Mid. Assoc, 1894, p. 63). 



DISTANCE OF THE SURGEON FROM THE PATIENT. 43 

What this distance may be will vary in different eyes. 
In general, it is limited by the size of the area in which 
the movement of light and shade is to be watched. The 
pupil fully dilated may be eight or ten millimetres in 
diameter, and movement across the whole width of such a 
pupil could be readily watched at a distance of 4 to 6 
metres. But the diameter of the visual zone of the pupil, 
the only area in which the movement is of practical 
importance, is commonly much less than this, say from 4 
to 6 millimetres ; and the movement of light across it can 
only be satisfactorily studied within the distance of two or 
three metres. Beyond one metre, however, the necessary 
inaccuracies of distance become usually of slight practical 
importance. 

In cases of aberration invading the central portions of 
the pupil and still more in cases of irregular astigmatism, 
the visual zone is considerably less in area than in the 
ordinary normal eye. In these cases, the test must be 
applied from a still shorter distance, often one-half or one- 
third of a metre, or even less. 

With the plane mirror it is easy to adopt any distance 
that suits the particular case. With the concave mirror 
any considerable variation in the distance requires a corre- 
sponding variation in the focus of the mirror used ; a mir- 
ror of shorter focus being employed when the distance 
between the observer and patient must be short ; and of 
longer focus if a greater distance is to be maintained. 

The reason for this is that if the concave mirror be 
brought too close to the observed eye it gives an immediate 
source of light relatively too large and too far in front of 
of the observer's position when at the point of reversal ; 
while if it be removed too far from the patient's eye, the 
diffusion of the light over a larger area is so rapid that it 
gives an illumination that is too feeble. These chat 
are much more rapid with the concave than with the 



44 CONDITIONS OF ACCURACY. 

plane mirror ; as :ne may readily demonstrate by holding 
both ::::rr:rs in his hand in the darkened room and reflect- 
ing areas of light npon a wall from various distances. I 
have elsewhere Jc the Am. JL^. Assoc., Sept. 4, 

1886 demonstrated the relations of the one to the other. 
In general the distance at which a concave mirror can be 
ttsecl t: best advantage is a little :ver ::::r times its fecal 
distance. 

F :r the majority :: t^ses then, a distance :: from : : 
to 2 metres is convenient for the plane mirror ; and one 
metre or a little '.ess for the concave mirror, having the 
usual focal distance of from 20 to 25 centimetres. 

Final Test. — When it is aesired to make the shadow 
test as accurate as possible, it is well to complete the test 
by placing before each eye lenses representing its suppcs-1 
correction, with such addition to the convex or diminution 
:: the concave spherical as shall bring the point of reversal 
to the greatest distance at which the movements of light 
and shadow can be satisfactorily studied in the particular 
eyes in : test:::: : and from that distance t: test the move- 
ment of light and shade, looking especially for uncorrected 
astigmatism, and comparing the one eye with the other for 
anv evidence of remaining inequalitv of refracticn. 



CHAPTER IV. 
REGULAR astigmatism. 

The essential fact of regular astigmatism is that in two 
different directions, at right angles to each other [the prin- 
cipal meridians] the curvature of the dioptric surfaces dif- 
fers, so that they exert unequal refractive power ; and that 
in all other directions, or meridians, the refractive power 
bears such a relation to the refractive power of these prin- 
cipal meridians, that it is only necessary to consider what 
happens in their direction. 

Two Points of Reversal. — In such an eye, the rays 
coming from the same point of the retina, and passing out 
through surfaces that refract unequally in different merid- 
ians, must leave the eye with different degrees of divergence, 
or convergence, in the directions of these different meridians. 
If after passing out the rays are convergent, or rendered so 
by passing through a convex spherical lens, they will be 
more convergent in one principal meridian than the other, 
and the point of reversal for one principal meridian will 
be nearer the eye, than the point of reversal for the other 
principal meridian. The position of each point of reversal 
gives the amount of myopia (either original or produced) 
in the principal meridian to which it belongs. The differ- 
ence between the amounts of myopia in the two principal 
meridians is the amount of astigmatism. The general plan 
of measuring astigmatism by skiascopy is to ascertain the 
point of reversal and measure the degree of myopia for 
each principal meridian separately; and then, by subtracting 

(45) 



46 REGULAR ASTIGMATISM. 

the one amount from the other, to find the amount of 
regular astigmatism. 

The Band-like Appearance. — This difference in the 
position of the points of reversal for the different meridians, 
gives rise to certain phenomena of great practical import- 
ance in skiascopy. It is true of the astigmatic as of the 
non-astigmatic eye, that, as the point of reversal is ap- 
proached, the image of the retina seen through the pupil 
becomes magnified (see page 29). And, it necessarily fol- 
lows that when the observer's eye is nearer to the point of 
reversal for one meridian than it is to, the point of reversal 
for the other meridian, the retinal image is more magnified 
in the direction of the principal meridian, to which the 
nearer point of reversal belongs. 

When the observer's eye is placed at the point of re- 
versal for one meridian, the retinal image becomes indefi- 
nitely magnified in the direction of that meridian, while 
comparatively little magnified in the direction at right 
angles to it. Bach point of the retina then appears in the 
pupil as a line running in the direction of that principal 
meridian, and the retinal light area, which consists of a 
number of these points, takes the form of an elongated band 
of light, running in the direction of the principal meridian 
which has its point of reversal at the observer's eye. This 
is the band-like appearance of the light in the pupil, char- 
acteristic of astigmatism ; and the less illuminated part of 
the pupil beside it is the " linear shadow " of Bowman. 
Figure 7 represents this appearance when the eye is placed 
at the point of reversal for one principal meridian, repre- 
sented about twenty degrees from the vertical ; and figure 
8 represents the appearance presented at the point of re- 
versal for the other principal meridian, twenty degrees 
from the horizontal. 

Its direction is always that of the principal meridian, at 
whose point of reversal it is seen ; and it is more pronounced: 



THE BAND-LIKE APPEARANCE. 47 

In proportion to the degree of astigmatism : 
The nearness of the observer's approach to the point of 
reversal : 

And the perfection of the focusing of the light upon 
the retina in the direction perpendicular to this principal 
meridian, that is, in the other principal meridian. 





Fig. 7. Fig. 8. 

In estimating astigmatism by skiascopy, two distinct 
things are to be done, which require different arrangements 
of the source of light. The first is to determine accurately 
the direction of the principal meridians by bringing out 
most distinctly this band-like appearance in the pupil, in- 
dicating the direction of one of these principal meridians ; 
the other being always, for regular astigmatism, at right 
angles thereto. The second thing to be done is to measure 
accurately the refraction in each of these principal merid- 
ians. 

The test proceeds at first as for myopia or hyperopia in 
a non-astigmatic eye, until a point of reversal is found. 
Then it is discovered that this point of reversal is only for 
the movement of light and shadow in one direction, and 
does not hold for movements at right angles to that direc- 
tion. The observer has now brought his eye to one point 
of reversal where the band-like appearance can be best per- 
ceived. But, as he has been working with the original 
source of light in the position most favorable for the meas- 
urement of hyperopia and myopia, the position that brings 



48 REGULAR ASTIGMATISM. 

the immediate source of light as close as possible to the 
mirror (see page ^j), he will probably see very little ap- 
pearance of the band in the pupil, even with the higher 
degrees of astigmatism, The reason for this is, that with 
the immediate source of light in this position, the light is 
most accurately focused on the retina in the direction that 
the band should take. And, in the direction at right angles 
to the band, the focusing is quite incomplete, so that the 
diffusion at what should be the sides of the band partly or 
entirely neutralizes the effect produced by the greater mag- 
nification of the retina in the direction of the band, which, 
otherwise, would cause the band-like appearance. 

In order to bring out this band-like appearance, it is 
necessary to make the focusing from side to side of the band 
as perfect as possible. And, to secure the perfect focusing 
in the principal meridian at right angles to the one in 
which the band is sought, the immediate source of light 
must be brought to the point of reversal for that other 
principal meridian. Ihe band-like appearance is most per- 
fectly developed when the observer's eye is at the point of reversal 
for one principal meridian, and the immediate source of light at 
the point of reversal for the other principal meridian. 




Fig. 9. 
In figure 9, the solid lines represent the vertical merid- 
ian of an astigmatic eye ; and the rays emerging, so turned 
in that meridian as to give the point of reversal at V. The 
broken lines represent the less curved horizontal meridian 
of the cornea, and the rays so turned in that meridian as to 
give a point of reversal at H. The dotted lines represent a 



THE BAND-LIKE APPEARANCE. 49 

plane mirror, P P, with the eye of the observer at V, and 
the light L pushed off from the mirror, so that the rays 
enter the eye as though they came from 17, and are per- 
fectly focused on the retina in the horizontal meridian, ren- 
dering most distinct the appearance of a vertical band. 

For illustration, suppose a case [which the student will 
do well to reproduce for actual study, either in the artificial 
eye or by lenses placed before the living eye] having com- 
pound myopic astigmatism, the vertical meridian of the 
cornea being 2 D. myopic and the horizontal meridian 1 D. 
myopic. When, with the plane mirror, the observer's eye 
is one-half metre from the observed eye, it will be at the 
point of reversal for the vertical meridian, and in a position 
to see a vertical band of light. But, if the source of light 
be placed as close to the mirror as possible, the rays from 
it will be the more accurately focused upon the retina in 
the vertical meridian and more diffused horizontally, so 
that the real form of the retinal light area will be rather 
that of a horizontal line or band. 

Now, from the observer's position, the retina is most 
magnified in the vertical direction, and this vertical magni- 
fication would cause a point of light on the retina to appear 
as a vertical band in the pupil ; but, with the light area 
really in the form of a horizontal band, the effect of the 
vertical magnification is largely neutralized, and the ap- 
pearance in the pupil may be quite indefinite. 

To bring out the band-like appearance : While keeping 
the observer's eye and mirror in the same position, the 
original source of light must be pushed off from the mir- 
ror one-half metre, the immediate source then retreats cor- 
respondingly behind the mirror, and approaches the posi- 
tion of the point of reversal in the horizontal meridian, 
one metre from the eye. 

With the light and mirror in this relation to the eye, 
the rays are perfectly focused upon the retina in the hori- 



50 REGULAR ASTIGMATISM. 

zontal meridian and diffused in the vertical meridian, so 
that the real form of the retinal area of light is a vertical 
line or band. This vertical line or band being viewed from 
the point of reversal of the vertical meridian (where it will 
be greatly magnified in the vertical direction and but 
slightly magnified in the horizontal direction), gives rise to 
the appearance of the most distinct vertical band of light 
in the pupil. And, under these conditions, the presence of 
the astigmatism and the direction of one of its principal 
meridians is most clearly and accurately revealed. 

Taking the same case and using the concave mirror at 
a distance of one metre, which is the point of reversal for 
the horizontal meridian, the appearance of a horizontal 
band of light in the pupil may be rendered most distinctly 
visible. But, in order to develop it clearly, it will be need- 
ful to bring the original source of light to such a posi- 
tion that the immediate source will be one-half metre in 
front of the mirror ; that is, one-half metre in front of the 
observed eye, at the point of reversal for the vertical merid- 
ian. For it is from this position the light will be most 
perfectly focused on the retina in the vertical meridian, 
while diffused in the horizontal meridian, and the greater 
horizontal magnification of the retina at the point of re- 
versal for the horizontal meridian wmere the observers eye 
is placed, will emphasize and increase the appearance of 
the horizontal band of light then thrown on the retina. 

Since, with the plane mirror, the immediate source of 
light is always back of the mirror, and cannot be brought in 
front of it, with it the direction of the band can only be accu- 
rately determined for the meridian whose point of reversal 
is nearest the eye. It is only with the eye and mirror at 
this point of reversal that one is able, with the plane mir- 
ror, to bring the immediate source of light to the other 
point of reversal. And, with the concave mirror, since the 
immediate source of light is always in front of the mirror, 



LIGHT AREA AT DIFFERENT DISTANCES. 51 

the band-like appearance can only be distinctly brought out 
in the meridian which has its point of reversal the farther 
from the eye, as only with the eye at that point of reversal 
can the immediate source of light, with the concave mirror, 
be brought to the other point of reversal. 

With either the plane or concave mirror, only the band 
in one of the principal meridians can be most distinctly 
developed. But it is unnecessary in practice to bring out 
the bands in both meridians, since, by knowing the direc- 
tion of one principal meridian, the other being always per- 
pendicular to it, is also known. 

The measurement of the refraction in either of the 
principal meridians of astigmatism, is quite similar to the 
measurement of refraction in hyperopia and myopia. To 
determine whether the movement of light in the pupil in a 
certain meridian is with or against the movement of light 
upon the face, it is necessary that the focusing of the light 
on the retina be as perfect as possible in that particular 
meridian. To secure this, the immediate source of light 
must be as close as possible to the position of the observer's 
eye. Hence, having determined the existence of the astig- 
matism and the direction of its principal meridians, the 
measurement in these meridians will proceed as the meas- 
urement of myopia or hyperopia. 

Changes in the Light Area at Different Distances. — 
In regular astigmatism, supposing the eye to be myopic in 
all meridians, or a convex lens placed before it sufficiently 
strong to over-correct the hyperopia in all meridians, the 
observer using a plane mirror and viewing the eye from 
different distances, will be able to recognize the following 
changes in the appearance and movement of the light in 
the pupil : 

From a position within the point of reversal of the 
more myopic meridian, the light will be seen to move with 
the light on the face, in all directions. As the observ- 



52 REGULAR ASTIGMATISM. 

^er's eye is withdrawn from the observed eye, and approaches 
the point of reversal for the more myopic meridian, the 
light area in the pupil becomes elongated in this meridian ; 
and, while the movement is still with the light on the face 
in all* meridians, it becomes more rapid in the direction of 
this elongation than in the direction perpendicular thereto. 

The observer withdrawing his eye still farther, on 
Teaching the point of reversal for the more myopic merid- 
ian, [ F, in figure 9,] is unable to distinguish the movement 
in this meridian, while the movement in the meridian at 
right angles to it is still with that of the light on the face. 

This point being reached, if the original source of light 
be pushed away from the mirror, so that its reflection, the 
immediate source of light approaches the point of reversal 
for the less myopic meridian, the form of the light in the 
pupil becomes a distinct band running in the direction of 
the more myopic meridian, readily seen to move from side 
•to side, but without perceptible movement in the direction 
of its length. 

Bringing the source of light back to its usual position 
close to the mirror, and withdrawing his eye still farther 
from the eye under observation, the observer again sees the 
movement of the light in the pupil in all directions. But 
in the direction of the most myopic meridian, it is now 
against the light on the face ; while in the meridian at right 
angles to this, it is still with the light on the face. The 
band-like appearance is now lost entirely ; the area of light 
in the pupil taking at one distance the same shape as 
though no regular astigmatism were present. 

But, as the point of reversal for the less myopic merid- 
ian is approached, elongation in the direction of that me- 
ridian may be noticed, and the movement of the light in 
that meridian with the light on the face becomes more rapid 
than the movement against the light on the face now seen 
in the more myopic meridian. When the point of reversal 



MOVEMENT OF THE BANDS IN ASTIGMATISM. 53 

for the less myopic meridian \H y figure 9] is reached, the 
movement in its direction ceases, but it is impossible, at 
this point (with the plane mirror), to bring out so distinct 
a band as was seen in the direction of the other meridian. 

Withdrawing still farther, the light in the direction of 
the less myopic meridian begins to *no"e agM"~, aq light 
on the face, at first very rapidly as compared with the 
movement in the more myopic meridian. But, as the ob- 
server withdraws farther from this second point of reversal, 
the difference in rate of movement in the two meridians 
becomes less noticeable. 

With the concave mirror, the same series of appearan- 
ces are presented, except that the directions of movement are 
reversed — the erect image seen from within the point of 
reversal giving movement of the light in the pupil against 
the movement of the light on the face, and against the 
mirror ; and the inverted image seen from beyond the point 
of reversal giving movement of the light in the pupil with 
the mirror and with the light on the face. (See page 26.) 
With the concave mirror the meridian in which it is possible 
to bring out the band-like appearance of the light most dis- 
tinctly is the meridian of less myopia. With such a mirror 
it will also be necessary to bring about the series of changes 
in the movement of the light area, which has been referred 
to, by changes of the lens placed before the eye, and not 
by changes in the observer's distance from the eye studied. 

Direction and Movement of the Bands in Astigma- 
tism. — The reason for the constant conformity of the di- 
rection of these bands of light to the principal meridians of 
refraction is obvious from their dependence on the magnifi- 
cation of the retina. That conformity sharply separates 
them from the somewhat similar appearance seen near the 
point of reversal in eyes free from astigmatism (page 31). 



54 REGULAR ASTIGMATISM. 

0~ ^ Th e apparent movement always 

sfff^sL "^ at right angles to their direction is 

0' : Wr \ I dependent on an optical illusion, of 

H |SL. p) / which one may satisfy himself by 

wP J J making a hole in the centre of a 

7--——^ ' sheet of paper, holding behind this 

^» \o/ hole the edge of a card, and moving 

it in a direction oblique to this edge. 

The motion will appear to be in a 

direction nearly or quite perpendicular to the edge seen. 

Thus, in figure 10, the real movement of the card be- 
hind the opening, or the band of light behind the pupil, 
may be in the direction o. But the movement will appear 
to be in the direction P p. 

Pendulum Movement. — In many eyes when the band 
of light is made to move across the pupil, it does not keep 
the same direction all the way across. But it appears on one 
side of the pupil to have a direction different from that 
which it has on the other side. Thus on the right side of 
the pupil, the band may have the direction of 75°, yet when 
moved to the left side, it has the direction of 105°. When 
moved from side to side, it appears to swing like the pen- 
dulum of a clock, around some fixed point. This fixed point 
may seem to be above the pupil, or below it, or to one side. 
R. D. Batten has pointed out (Ophthalmic Review Jan., 
1897) that this behavior of the band of light resembles what 
is seen in conical cornea (see page 64) ; except that with 
the pendulum movement, the apex of the cone is entirely 
beyond the edge of the cornea. He has suggested that this 
condition be called Conical Astigmatism. The pendulum 
movement is liable to mislead the surgeon as to the direction 
of the meridians of astigmatism. This must be guarded 
against by carefully noting its direction, at the center of 
the pupil. 



CHAPTER V. 

ABERRATION AND IRREGULAR ASTIGMATISM. 

In astigmatism, strictly regular, though the refraction 
differs in different meridians, in any given direction or 
meridian it is the same at all parts of the pupil. In aber- 
ration and irregular astigmatism, the refraction differs in 
different parts of the pupil, even in the same meridian. 
All eyes present variations of this kind ; and these varia- 
tions constitute an obstacle to the measurement of refrac- 
tion, by skiascopy or by any other method. 

Appearances of Irregular Astigmatism. — To the be- 
ginner with skiascopy, these constitute the most serious 
obstacle he has to encounter. By one who has thoroughly 
mastered the principles of the test and become familiar 
with the various appearances of light and shade in the 
pupil, mistakes due to aberration or irregular astigmatism 
are readily avoided ; and to the experienced skiascopist is 
revealed the reason for uncertainty in the results obtained 
by other methods, or of failure to secure perfect vision on 
account of these defects. 

If we suppose two parts of the pupil, one of which has 
its point of reversal at the observer's eye, while the other 
is at a considerable distance therefrom, the illumination 
of the former will be the more feeble, of the latter the more 
brilliant ; the movement of the light in the former, if per- 
ceptible, will be rapid, in the latter, slow. If one watches 
two parts of the pupil, one of which has its point of rever- 
sal back of the observer's eye, and the other in front of it ; 
in the former the light will have a direct and in the latter 
an inverted movement. 

(55) 



56 ABERRATION AND IRREGULAR ASTIGMATISM. 

With the irregular astigmatism due to preceding cor- 
neal inflammation, or to the changes in the refraction of the 
lens that sometimes precede cortical cataract, the pupil ap- 
pears broken up into a considerable number of distinct 
areas, each of which has its separate movement of light and 
shadow, constituting the typical ophthalmoscopic or skia- 
scope picture of irregular astigmatism. The appearance 





Fig. ii. Fig. 12. 

caused by irregular astigmatism following corneal disease 
is shown in figure 11. That due to changes in the lens 
such as may precede cortical senile cataract is shown in 
figure 12, in which the black lines represent fixed spicules 
of actual opacity, while the other parts of the pupil indicate 
merely refractive differences, and change from light to dark, 
or dark to light, as the inclination of the mirror is varied. 
Some such appearance is sometimes presented by young 
persons, indicating a congenital defect which may not 
noticeably increase in many years. 

If the differences of refraction in the different parts of 
the pupil are slight — that is, if the aberration or irregular 
astigmatism is of low degree — -these differences of illumina- 
tion and movement will not be perceptible until the ob- 
server brings his eye close to the point of reversal. But at 
the point of reversal, they become perceptible and consti- 
tute a striking phenomenon in almost all eyes ; and, to the 
observer who does not understand their significance, one 
that is extremely confusing. In the nature and arrange- 



SYMMETRICAL ABERRATION. 57 

ment of its irregular astigmatism, every eye is peculiar. 
The varieties of play of light and shade that are obtainable 
as the point of reversal is reached, are as numerous as the 
eyes examined. Even the two eyes of the same individual 
differ. 

The only practical way to deal with irregular astig- 
matism by skiascopy is to understand thoroughly the gen- 
eral optical principles of the test, and apply them, so far as 
may be needful, in the individual case. Certain peculiar 
forms of variation of the refraction of the eye in different 
parts of the pupil are, however, of sufficient constancy, 
regularity and practical importance, to warrant their sepa- 
rate classification and study. The most important of these 
is the regular, or symmetrical, aberration cf the eye. 

Symmetrical Aberration. 1 — This is an error of the 
refraction of the eye which causes the rays of an incident 
pencil falling on the same meridian of the cornea, but at 
different distances from the axial ray, to meet at different 
distances behind the cornea, while rays piercing different 
meridians of the cornea, at the same distance from the axial 
ray, intersect it at the same point. It is a defect similar to 
the monochromatic aberration of convex and concave spher- 
ical lenses. It is readily recognizable in almost all eyes by 
skiascopy. In the majority of cases, it is in the same direc- 
tion as ordinary spherical aberration ; that is, near the 
margin of the pupil there is a stronger lens action than 
near the centre — the rays entering through the margin 
are brought to a focus first, the rays entering nearer the 
centre being focused farther back. This is called posit ice 
aberration. 

In a certain proportion of cases, however, the defect is 
in the opposite direction ; the rays passing near the centre 
of the pupil being brought first to a focus, and those pass- 

1 For an account of this error of refraction see paper by the author. Trans. 
Amer. Ophthalmological Society, 188S, p. 141. 



58 ABERRATION AND IRREGULAR ASTIGMATISM. 

ing through the periphery being focused farther back. The 
centre of the pupil has the stronger, and the periphery of 
the pupil the weaker, lens action. This is negative aberra- 
tion. 

The Visual Zone. — The variation of refraction, how- 
ever, does not usually proceed regularly from the centre of 
the pupil to the margin. But, as with spherical lenses, 
and to a greater degree, the central refraction is compara- 
tively uniform over a considerable area ; and, towards the 
margin the change of refraction becomes progressively more 
marked. This area in the centre of the pupil of compara- 
tively uniform refraction is the usual visual zone. It is the 
portion of the pupil that is of practical importance for pur- 
poses of distinct vision. Its size varies considerably. Some- 
times it includes almost the whole of the dilated pupil, in 
other eyes an extremely small area near the centre of the 
pupil will be regular, and the remainder of the pupil use- 
less for accurate vision. If a high degree of irregular 
astigmatism be present, the visual zone, instead of being a 
central area of considerable size, will often be some particu- 
lar portion of the undilated pupil, which happens to have 
the most regular curvature. 

In any case, for the correction of ametropia, it is the 
behavior of the light and shade in the visual zone which 
has to be studied. Its behavior elsewhere may be disre- 
garded. It is often much easier to watch the movement of 
light and shade in some other portion of the pupil — some 
part of the extra-visual zone. And, if the observer does 
not understand their relative importance, he will be apt to 
fix his attention on this latter, and be led away from the 
true refraction of the eye he is examining. This is the 
more likely to happen, because in that part of the pupil, 
which has its point of reversal at, or near, the observer's 
eye, the direction of the movement of light and shade is 
difficult to see, while in other portions, the movement is 
more striking. 



THE APPEARANCES OF POSITIVE ABERRATION. 59 

The Appearances of Positive Aberration. — The ap- 
pearances presented by an ordinary case with positive aberra- 
tion may be considered in the order in which they will be 
developed with the plane mirror, the observer starting to 
examine the eye from within the point of reversal for the 
most myopic part of the pupil, and gradually withdrawing 
his eye until it is beyond the point of reversal for the least 
myopic part of the pupil. From the first position, the light 
area in the pupil is seen to move with the light on the face 
entirely across the pupil ; its motion in the edges of the 
pupil being more rapid than in the centre. If, now, the 
observer's eye is withdrawn to the point of reversal for the 
margin of the pupil, there appear in the margin points in 
which no movement of the light can be seen. Some of 
these may be points of stationary light, and others, points 
of stationary shadow. 

As the observer's eye is still farther withdrawn, the 
points of stationary light run together and form a complete 
ring of light in the periphery of the pupil, shown in figure 
13, which is presently seen to have an inverted motion — to 
move against the light on the face. Within this is a ring 





Fig. 13. Fig. 14. 

of comparative shadow where the movement is swift, and 
difficult or impossible to recognize ; and still within this 
lies an area of light — what remains of the light area first 
seen, now considerably reduced in size — which still moves 
with the light on the face. 



60 ABERRATION AND IRREGULAR ASTIGMATISM. 

As the observer draws still farther back, this area of 
light at the centre of the pupil, as shown in figure 14, 
grows smaller, and its movement more difficult to certainly 
distinguish. The ring of comparative shadow around it 
encroaches upon it, and the ring of light in the margin of 
the pupil in turn encroaches upon the shadow, and becomes 
brighter and its movement more readily noticeable. 

Withdrawing still farther, the point of reversal for the 
centre of the pupil is reached. The central area of light 
becomes faint and its movement ceases to be noticeable, 
the ring of feeble illumination surrounding it having swal- 
lowed it up. But, around this feeble light area, the ring 
of inverted movement has now grown broad and distinct. 
And, as the observer withdraws still farther, this ring of 
inverted movement closes in until it occupies the whole of 
the central area, and the observer sees an area of light 
moving across the whole pupil, having an inverted move- 
ment — that is, against the mirror or light on the face. 

The movements of these erect and inverted light areas 
in the pupil are illustrated by figures 15 and 16. Figure 
15 shows the plane mirror turned to the left, or the concave 
mirror turned to the right, the central erect area being dis- 
placed toward the left, and the peripheral inverted area to- 
ward the right of the space it occupies. Figure 16 repre- 
sents the light areas displaced in the opposite directions by 
an opposite inclination of the mirror. 

With the concave mirror, a similar series of changes 
may be brought about by placing before the ej T e successive 
strengths of the lenses, beginning with the weakest convex 
or strongest concave. The first should allow the points of 
reversal for all parts of the pupil to be back of the observer, 
and the successive changes bring these points closer and 
closer to the observed eye until all are in front of the ob- 
server. The movement is at first against the light on the 
face. Then appears the ring of illumination and swift 



THE APPEARANCES OF POSITIVE ABERRATION. 



61 



movement in the margin of the pupil with the light on 
the face. The central area of light is then encroached 
upon by the ring of faint illumination, and this in turn by 





Fig. 15. 



Fig. 16. 



a ring of more brilliant illumination in the margin, moving 
with the light on the face, which latter finally occupies the 
whole area of the pupil. 

If the point of reversal be approached from the oppo- 
site direction, that is, starting with the observer's eye beyond 
it, we have, with the plane mirror, at first, inverted move- 
ment across the whole pupil. Then, as the point of re- 
versal for the centre of the pupil is approached, the light 
in the central zone becomes feeble and its movement indefi- 
nite. When the point of reversal for that part is passed, 
there appears, in this central zone, an erect movement of 
light and shade, at first rapid and hard to see, but growing 
slower, gaining in distinctness, and occupying a larger and 
larger part of the pupil as the patient's eye is approached, 
until, finally, it occupies the whole area. 

With the concave mirror, starting with the strongest 
convex or weakest concave lens, the movement is first with 
the mirror throughout the pupil, then, as the lens is 
changed, it becomes indefinite at the centre ; presently it is 
against that of the mirror at the centre, while still with it 
at the margin ; and, with still weaker convex lenses, or 
stronger concaves, it becomes against the mirror through- 
out the whole pupillary area. 



62 ABERRATION AND IRREGULAR ASTIGMATISM. 

Appearances of Negative Aberration. — With nega- 
tive aberration, the series of changes is apt to be less regu- 
lar and complete, and the picture presented by the pupil is 
less characteristic. But the succession of appearances is 
the reverse of what has been described for positive aberra- 
tion. 

With a plane mirror starting closer to the patient's eye 
than the point of reversal for the most myopic part of the 
pupil, the movement is with that of the light on the face 
throughout the whole pupil. As the observer's eye is with- 
drawn to a greater distance, this movement becomes indefi- 
nite, and the light feeble near the centre of the pupil. 
Presently, the movement at the centre of the pupil is lost, 
while still quite distinctly with that of the light on the face 
in the irregular ring-shaped area of the periphery. With- 
drawing still farther from the eye, the inverted movement 
at the centre of the pupil becomes distinctly visible, and 
the direct movement near the margin becomes more and 
more encroached upon and less and less distinct, until 
finally all erect movement is lost and w^e have only the in- 
verted movement, which extends across the whole pupil. 
Before the erect movement entirely disappears, it is apt to 
break up into small areas detached from one another by 
spaces of comparative shadow, but each presenting some 
remnant of the erect movement. 

With the concave mirror, starting with a convex lens so 
weak, or a concave lens so strong, that the point of reversal 
is back of the observer, we have the movement against the 
light on the face throughout the pupil. The strengthening 
of the convex lenses or the weakening of the concaves, so 
as to bring the point of reversal close to the observer's eye : 
causes, first, the fading of the light and the indefiniteness 
of its movement in the centre of the pupil, then the move- 
ment, with the light on the face, at the centre, and the area 
of this movement extending until it includes the whole of 
the pupil. 



APPEARANCES OF NEGATIVE ABERRATION. 63 

Approaching the point of reversal from beyond it, we 
have, with the plane mirror, inverted movement through- 
out the whole pupil, giving place to indistinctness first at 
the margin. Then the indirect movement confined to the 
central area of the pupil and direct movement appearing at 
certain parts of the marginal area. This direct movement 
becomes more distinct and its area increases as the patient's 
eye is approached, until, at the point of reversal for the cen- 
tre of the pupil, all inverted movement is lost, and the erect 
movement is seen in all parts of the pupillary area. 

With the concave mirror, starting with the point of 
reversal between the observer and patient, and removing it 
successively farther from the patient, by the use of weaker 
convex or stronger concave lenses, we have first the move- 
ment with the light on the face throughout the pupil, 
then indefinite at the pupillary margin, changing, in 
turn, to movement against the light on the face. The area 
of this peripheral movement then encroaches upon the 
central area until that is obliterated, and the movement 
against the light on the face occupies the whole width of 
the pupil. 

While the order of their development remains the 
same, the exact character of the appearances presented var- 
ies considerably with the degree of aberration. Generally, 
in the higher degrees, the areas of light occupy the greater 
part of the pupil and the area of feeble illumination sepa- 
rating them is comparatively narrow. While in very low 
degrees of aberration, the area of feeble illumination is 
broad, and it may be difficult to recognize more than one 
of the light areas at one time. That is, when the area of 
erect movement is visible, the remainder of the pupil is 
occupied by the area of feeble illumination ; and when the 
area of inverted movement is developed, the area of feeble 
illumination so encroaches upon the area of direct move- 
ment that the latter can no longer be identified. 



64 ABERRATION AND IRREGULAR ASTIGMATISM. 

In some eyes, the variation of refraction from point to 
point, which constitutes symmetrical aberration, is almost 
or entirely confined to the periphery of the pupil. In these 
eyes the appearances characteristic of aberration are hard 
to develop. 

Appearance of Conical Cornea. — In other eyes, an 
opposite condition is present. The variations of refraction, 
instead of being confined to the periphery of the pupil, 
encroach upon the normal visual zone, confining it to a 
very narrow area. In these eyes, the skiascopic appear- 
ances of aberration are striking and characteristic, and one 
of them is that which has been regarded as peculiar to con- 
ical cornea. 

The error of refraction produced by conical cornea is 
a high degree of negative aberration. At the apex of the 
cone, the curve is sharp, causing, usually, very high myopia 
in the corresponding part of the pupil. The sides of the 
cone, on the other hand, are comparatively flat, causing 
diminished myopia as the region of the apex is departed 
from and sometimes running into hyperopia near the edge 
of the pupil. 

If the observer's eye be placed somewhere near the 
point of reversal for the periphery of the pupil, the move- 
ment of light in that portion of the pupil will be rapid, but 
the movement in the portion of the pupil corresponding to 
the apex of the cone will be slow. On account of the high 
myopia, the point of reversal for this part of the pupil is 
very close to the eye, and, generally, many dioptres removed 
from the observer's eye. The movement of light in the 
pupil, then, is slow near the centre and rapid towards the 
periphery, causing the area of light to appear to wheel 
around a fixed point corresponding to the apex of the cone. 
The light area is first seen on one side of the pupil, then on 
the other, but always rests upon the central fixed point. 

In certain positions of the light, the form of this area 



APPEARANCE OF CONICAL CORNEA. 



m 



will be somewhat triangular, its base resting on the margin 
of the pupil and its apex at the apex of the corneal cone. 
Sometimes the triangle covers almost half of the pupil, in 
other conditions of light it is considerably narrower, but 
the constant and characteristic phenomena is the wheeling 
of the light area about the fixed point at the apex. 

This is shown in figures 17 and 18, which represent 
the appearances of the pupil with the mirror inclined in 
opposite directions. 

It was for the detection of these appearances, to which 
attention was called by Bowman, in 1857, that the test was 
first employed. Bowman mentions that he was able by 
means of it to detect low degrees of conical cornea, which 
could not be detected in any other way. It is certain that 
among those cases that have been classed as low degrees of 
conical cornea, on account of their presenting such appear- 





Fig. 17. 



Fig. 18. 



ances, a considerable proportion were not of conical cornea 
at all, but were cases of high aberration from other forms 
of defect in the dioptric surfaces. 

The appearances in question occur in all cases of high 
aberration. Where the aberration invades the central por- 
tion of the pupil, and is not confined to the periphery, the 
phenomena are quite as striking and characteristic, as in 
cases of conical cornea. And cases of high positive aberra- 
tion are more common than cases of true conical cornea. The 
conditions for their recognition are that the observer's eye 



66 ABERRATION AND IRREGULAR ASTIGMATISM. 

shall be comparatively near the point of reversal for the 
margin of the pupil, and comparatively far removed (esti- 
mating by dioptres) from the point of reversal for the cen- 
tre of the pupil. By careful management of the light and 
relative position of observer and patient, something of such 
an appearance can be demonstrated in the majority of eyes. 

Like the band-like appearances of the light in astig- 
matism, those of conical cornea reveal the presence of the 
condition and the location of the apex of the cone, but be- 
yond this, they are of little value. The measurement of 
the difference of refraction between the margin and the 
centre of the pupil, or the measurement of the refraction in 
the portion of the pupil best suited to purposes of vision, 
must be accomplished by the same application of skiascopy 
as serves to measure the amount of hyperopia or myopia in 
an eye free from astigmatism and aberration. 

The series of movements presented in positive aberra- 
tion can be well studied in one of the numerous forms of 
artificial eye, in which spherical lenses are used to repre- 
sent the dioptric surfaces ; and it is well by such study to 
become thoroughly familiar with them. They may, of 
course, be studied in living eyes presenting positive aberra- 
tion ; but, in many of these, the appearances are not so 
typical, and regular in order of sequence, as with the ordi- 
nary strong spherical lenses. The appearances presented 
by negative aberration can only be studied in eyes in which 
this condition of the refraction is present, but their recog- 
nition and observation will be comparatively easy to one 
who has mastered the corresponding appearances of positive 
aberration, and who understands the optical conditions on 
which they depend. 

Scissors-like Movement. — A special form of irregular 
astigmatism exists, of sufficiently frequent occurrence and 
striking character to merit special description. It is also 
of some practical importance. In it, one portion of the 



SCISSORS-LIKE MOVEMENT. 67 

pupil, as an tipper or lower half, is more myopic in a cer- 
tain meridian than is the other part of the pupil. This 
causes an inverted movement of light in the one portion of 
the pupil, while there is an erect movement in the other. 
These two areas are distinct, and separated by an interme- 
diate zone of feeble illumination. As the light is made to 
move back and forth in the proper direction, the two areas 
of light in the pupil are seen alternately to approach and 
separate, narrowing or widening the intermediate zone. As 
the areas, under these circumstances, are generally band- 
like, or have comparatively straight margins, the effect 
is similar to that of the opening and closing of a pair of 
scissors. These appearances are represented in figure 19, 
which shows them with the mirror so turned as to separate 
the two areas ; and figure 20, which represents theni brought 
together by an opposite inclination of the mirror. Suppos- 
ing the upper part of the pupil to be more myopic, figure 
19 corresponds' to the plane mirror facing down or the con- 
cave mirror facing up ; and figure 20 shows the plane mir- 
ror facing up or the concave mirror facing down. 





Fig. 19. Fig. 20. 

The relative size of the two areas will depend on the 
distance of the observer from the eye or upon the strength 
of the lens employed. As the observer withdraws to a 
greater distance, or the convex lens is made stronger, or the 
concave lens is made weaker, the area of inverted move- 
ment encroaches upon the zone of feeble illumination sep- 



68 ABERRATION AND IRREGULAR ASTIGMATISM. 

arating the areas of light and the area of erect movement 
diminishes. As the observer comes closer to the eye, or 
the convex lens is made weaker or the concave lens stronger, 
the area of inverted movement diminishes. Always the 
observer's eye is at or near the point of reversal for the por- 
tion of the pupil occupied by the intermediate zone of 
feeble illumination; and, in making the determination of 
the refraction for practical purposes, care must be taken to 
see that this zone occupies a portion of the pupil that is 
available when the pupil is contracted, as under ordinary 
conditions of illumination and near work. 

The scissors-like movement may be produced in an 
artificial eye by placing the lens which represents the diop- 
tric surfaces, so that the light passes through it obliquely. 
It may also be developed in most eyes by applying skias- 
copy in some direction at a considerable angle to the optic 
axis. Its presence in the eye indicates obliquity or imper- 
fact centreing of one or more of the dioptric surfaces. 
Probably it is often due to some obliquity in the position 
of the crystalline lens. Perhaps, because of such obliquity, 
this appearance of light and shadow in the pupil is apt to 
co-exist with a considerable degree of regular astigmatism, 
which, on account of it, becomes more difficult to recognize 
and measure than it would otherwise be. Eyes presenting 
it, therefore, demand special care and attention on the part 
of the observer, to develop the best vision they are capable 
of with correcting lenses. 



CHAPTER VI. 

PRACTICAL APPLICATION WITH THE PLANE MIRROR. 

Position- and Arrangement of Light. — The room being 
thoroughly darkened, the patient and surgeon take posi- 
tions facing each other at a distance of about one metre, 
with the original source of light close to the surgeon on 
the side of the eye he desires to use, that is on the right if 
he intends using his right eye for the test. He can really 
see the movement of light and shade in the pupil with but 
one eye at a time ; yet he will find it more pleasant to work 
with both eyes open if he once learns to do so. The source 
of light should be freely movable from fifteen centimetres 
in front of the patient's face to over a metre away, a move- 
ment obtainable with a double'jointed bracket of over one- 
half metre total length. The light is covered from the 




M 

V O 

Fig. 21. 

patient's face, and also from the surgeon's except at the 
aperture of about five millimetres opposite the brightest 
part of the flame. The arrangement of the surgeon and 
patient with reference to the light is shown in Fig. 21, in 
which L represents the light, M the mirror, and and P 
the eyes of the observer and patient. 

(GO) 



70 PRACTICAL APPLICATION WITH THE PLANE MIRROR. 

The mirror is held so that with the eye behind it the 
surgeon can watch, through the sight hole, the movement 
of light on the patient's face ; and turned until the area of 
light that it reflects falls upon the eye to be tested. If dif- 
ficulty is experienced in properly directing the light, the 
surgeon may hold his hand a few inches in front of the 
mirror and upon it find the light area and get it properly 
directed towards the patient's eye. If the mirror be large 
it is necessary that the central portion of the light area be 
made to fall upon the patient's eye, the centre being marked 
by the spot of feeble illumination, corresponding to the 
sight hole of the mirror. 

With the light properly directed, the pupil appears to be 
partly or wholly occupied by a red glare, the light area with 
which skiascopy is especially concerned. In first attempt- 
ing the test, care must be taken to discriminate clearly be- 
tween this general red glare and the reflection from the 
cornea or from the surfaces of any lens that may be placed 
before the eye. These reflections have the same color as 
the light used for the test. The reflection from the cornea is 
small and brilliant, a mere point of light, if the room be 
thoroughly darkened and the original source of light prop- 
erly shaded. The reflections from the lens employed are 
larger and more confusing. They may be avoided by tilt- 
ing the lens slightly, which^ causes them to pass off to the 
periphery, leaving the centre of the lens free from reflection. 

Hyperopia. — If the mirror be rotated about a vertical 
axis, that is if it be made to turn more to the right or left, 
the area of light in the pupil will be seen to move with the 
light on the face to the right or left as the inclination of 
the mirror changes. If the rate of movement be slow, the 
hyperopia is of high degree, if more rapid, it is lower. 

A convex lens is now to be placed before the eye, and 
this rate of movement of light in the pupil is the guide to 
the probable strength of lens required. If the observer has 



HYPEROPIA. 71 

not sufficient experience with skiascopy to judge in this way 
about the strength of the lens required, he will save time 
by placing before the eye rather a strong lens, one of say 
5 D. With this the light is again thrown upon the eye, 
and if the lens be 'not sufficient to correct the hyperopia 
present, the movement of light in the pupil will still be 
with that of the light on the face. In this case a still 
stronger lens must be used. This strengthening of the 
convex lens before the eye is continued until one is found 
which causes the reversal of the apparent movement of 
light in the pupil — until the light in the pupil moves 
against the light on the face. 

Then the surgeon is to approach the patient, mean- 
while rotating the mirror and watching for the nearest point 
at which he still sees the inverted movement in the visual 
zone. Near this point, the illumination of the pupil be- 
comes quite feeble ; and the movement, being rapid, requires 
the closest watching. Approaching still nearer to the 
patient the light in the visual zone is seen to move with the 
light on the face, and the greatest distance at which this 
can be distinguished is to be noted. Between these two, 
the least distance of inverted movement, and the greatest 
distance of direct movement lies the point of reversal. 
But, for reasons given page 39, it is better to take the lat- 
ter, the greatest distance at which direct movement can be 
perceived as the point sought. 

The distance from the surgeon's eye to the patient's 
eye is then measured. It is the focal distance of the amount 
of myopia produced by the convex lens employed. That 
amount is to be subtracted from the total strength of the 
lens to ascertain the proportion of its strength which has 
been necessary to correct the existing hyperopia. 

Having made such a determination of the refraction 
and having repeated the various observations until no 
doubt is left as to their correctness, the lens before the eye 



72 PRACTICAL APPLICATION WITH THE PLANE MIRROR. 

is to be changed for one sufficiently weaker to cany the 
point of reversal to as great a distance, as the size of the 
visual zone will allow for the accurate determination of the 
movements of light and shade within it. At this distance 
the estimate of the ametropia is to be completed. 

For example : Suppose the eye under examination to 
have hyperopia of 3 D. When the 5 D. lens is placed be- 
fore it, the point of reversal will be brought to one-half 
metre. As the surgeon's eye is made to approach that of 
the patient, the inverted movement in the visual zone will 
cease when they are about 60 centimetres apart. Going 
still closer, the erect movement will be distinguished at 
about 40 centimetres. These observations are to be repeated 
until the surgeon makes sure that the point of reversal lies 
somewhere between 40 and 60 centimetres. The 5 D. lens 
is then replaced by the 4 D. lens. Repeating the test, the 
inverted movement is seen as near the eye as one and one- 
quarter metres and the direct movement almost as far away 
as one metre. This locates the point of reversal at about 
1 metre from the eye, and determines the myopia caused by 
a 4 D. convex lens to be 1 D., and the refraction of the eye 
to be 4. D.-i D.=3 D. of hyperopia, with less than 0.25 D. 
of possible error either way. 

Myopia. — In myopia the first rotation of the mirror 
will usually cause a movement of light in the pupil against 
that of the light on the face. The surgeon then approaches 
the patient, continuing the movement of the mirror and 
watching the apparent movement of light in the pupil, 
until this apparent movement becomes rapid and indefinite 
and presently is entirely lost. Approaching still closer to 
the patient's eye, the movement of the light area in the 
pupil again becomes distinct, but is now with the move- 
ment of the light on the face. Drawing back again, the 
surgeon notes the greatest distance at which this erect 
movement can be observed, and then again the shortest 



MYOPIA. 73 

distance at which the inverted movement is distinguishable, 
and takes a point midway between these to be the point of 
reversal. 

The distance of this point of reversal from the patient's 
eye is the focal distance of the lens that will be required to 
correct the myopia. To complete the test, however, a lens 
about i D. weaker than this is placed before the eye to 
bring the point of reversal to the distance of one metre ; and 
the test is repeated, the surgeon noting carefully the great- 
est distance at which the erect movement is visible, and 
the shortest distance at which the inverted movement is 
perceived, always in the visual zone. The distance of the 
point of reversal as thus determined is the focal distance of 
the lens required to correct the remaining myopia. The 
strength of such a lens, added to the strength of the lens 
already before the eye, gives the total amount of myopia 
present. 

Suppose the eye to be 6.5 D. myopic. With the first 
test the inverted movement will be perceived up to about 
eight inches from the patient's eye ; and at five or six inches 
from the eye an erect movement will begin. From this r 
the surgeon may assume that the myopia is about 7 D. 
[focal distance 6% inches] and place before the eye, 
for the more accurate test, a concave 6 D. lens. On 
trying the movement of light in the pupil through this 
lens, it will be found at the distance of one metre to be 
with that of the light on the face. The surgeon then with- 
draws still farther from the patient until the direct move- 
ment becomes indistinguishable and at two metres is entirely 
lost. Drawing back still farther from the patient, he might 
in a favorable eye be able to distinguish the inverted move- 
ment in the pupil, and in this way fix the point of reversal 
at a distance of two metres, indicating with great accuracy 
an uncorrected myopia of 0.5 D. 

Often, however, the distance of two metres will be found 



74 PRACTICAL APPLICATION WITH THE PLANE MIRROR. 

so great that it is difficult or impossible from such a distance 
to be sure of the movement in the visual zone. In such a 
case the 6 D. lens will need to be replaced by a weaker lens 
as a 5.5 D., with which the erect movement will be seen to 
almost a metre, and the inverted movement will begin a 
few inches beyond that point. 

If the myopia be very low, the first inspection of the 
pupil without a lens may show a movement of light in it 
with the light on the face. In such a case, the surgeon 
will draw back as far as he can readily distinguish the 
movement of light in the visual zone. If the movement 
still appears to be with that of the light on the face, he will 
place before the eye a convex lens, and with it determine the 
point of reversal as for a case of hyperopia. The final 
result of testing, however, will show that the myopia caused 
by the lens is greater than the strength of the lens, and, 
therefore, that some myopia must have been present before 
the lens was placed in front of the eye. 

For example : Suppose that before reaching that dis- 
tance of two metres the erect movement in the pupil 
becomes indistinct, and that the visual zone, where the 
movement must be watched, is so small that beyond this 
the direction of movement in it cannot be recognized with 
certainty. A 0.5 D. convex lens being placed before the 
eye is found to cause an inverted movement beyond 125 cen- 
timetres, and to confine the erect movement to within 85 
or 90 centimetres of the eye. The point of reversal then, 
is at one metre. The amount of myopia corresponding to 
this is 1 D., of which 0.5 D. was the amount originally 
present in the eye. 

Emmetropia. — On first inspection, without a lens, the 
surgeon sees an erect movement in the pupil, the rapidity 
of which indicates that if there be hyperopia it is of low 
degree. Drawing back from the patient's eye as far as pos- 
sible, however, this erect movement still continues. He 



REGULAR ASTIGMATISM. 75 

places before the eye under observation a convex lens of i 
or 2 D., and viewing the movement of light in the pupil 
through this lens, finds where the inverted and the erect 
movement come together. On measuring the distance of 
this point of reversal from the patient's eye, he finds that 
it exactly corresponds with the focal distance of the lens he 
has been using. That is, the lens has caused myopia just 
equivalent to its own strength, showing that before they 
passed through the lens, the rays emerging from the cornea 
were parallel. 

Regular Astigmatism. — Whether it be known that 
the eye under examination is astigmatic or not, the test 
will proceed at first as for simple hyperopia or myopia. 
Sometimes if the astigmatism be high and one meridian 
nearly emmetropic or slightly myopic, the first inspection, 
without any lens, will reveal an unmistakable baud of light, 
or that there is erect movement in one meridian and in- 
verted movement in another, or that the movement of 
light in the pupil is more rapid in some one meridian than 
in the meridian at right angles to it, indicating that these 
meridians have different points of reversal, and that the 
surgeon is nearer the point of reversal for one than for the 
other, or is between the two. 

But, commonly, the first appearance will give no posi- 
tive indication of the presence of astigmatism, and the test 
goes on until a point of reversal is found. Then, on trying 
the movement of light and shade in different meridians, as 
should always be done from the neighborhood of the point 
of reversal, it is discovered that it is the point of reversal 
for only one meridian ; and that for the meridian at right 
angles to that one, there is a distinct movement of the light 
either erect or inverted. 

If the movement still noticeable from the point of 
reversal first discovered be an inverted movement — against 
the light on the face — the surgeon should bring his eve 



76 PRACTICAL APPLICATION WITH THE PLANE MIRROR. 

still closer to the patient until this inverted movement 
ceases. He will then be near the point of reversal for the 
meridian in which the inverted movement was before 
noticed, and will be able to see in the other meridian an 
erect movement. 

Such a lens is now to be chosen and placed before the 
eye as will bring this point of reversal for the more myopic 
meridian — the point of reversal from which an erect move- 
ment is seen in the other meridian — to a convenient distance 
from the eye. The surgeon's eye is placed as nearly as pos- 
sible at this point of reversal. Then the original source 
of light [which has up to this stage of the test accompanied 
the mirror in its movements to or from the patient's eye] 
is pushed away from the mirror, and while it is pushed 
away, the mirror is rotated and the light area in the pupil 
watched. This light area will now be seen to assume the 
band-like appearance characteristic of astigmatism. 

At a certain distance this band-like appearance will be 
most distinct. With the source of light nearer the mirror 
or farther from the mirror, it will be less characteristic. 
The distance of the light from the mirror at which the 
band becomes most distinct is the distance between the two 
points of reversal. The surgeon's eye (with the mirror) is 
now at the point of reversal for the more myopic meridian, 
and the immediate source of light is at the point of rever- 
sal for the less myopic meridian. (See page 48.) 

With the light in this position, the direction of the 
band is to be carefully studied and noted as the direction 
of one of the principal meridians of astigmatism. It is the 
direction of the axis of the convex cylinder that will cor- 
rect the astigmatism. The other principal meridian will, 
of course, be perpendicular to this. 

Having now fixed the direction of the principal me- 
ridians of astigmatism, the surgeon should again bring the 
original source of light as near to the mirror as possible, 



REGULAR ASTIGMATISM. 77 

and proceed to measure the refraction, first in the one prin- 
cipal meridian and then in the other, just as he would 
measure the refraction in a case of hyperopia or myopia. 
The difference between the refractions of the two principal 
meridians being the amount of astigmatism present. 

To measure the refraction in a certain meridian the 
light is made to move on the face, and on the retina, in the 
direction of this meridian by rotating the mirror about an 
axis perpendicular to it. Thus for the vertical meridian 
the light is made to move vertically by turning the mirror 
about a horizontal axis. For the horizontal meridian the 
light is made to move horizontally, by turning the mir- 
ror about a vertical axis. Great care is necessary in the 
higher degrees of astigmatism to make the movement. con- 
form accurately to the meridian to be tested, since any 
oblique movement will appear (see page 53) as though per- 
pendicular to the band. 

When the astigmatism is of very low degree, 0.5 D. or 
less, it becomes correspondingly difficult to distinguish be- 
tween the points of reversal for its principal meridians. 
The band-like appearance of the light in the pupil becomes 
less characteristic, and there is no space between the two 
points of reversal where an erect movement can be obtained 
in the direction of one meridian, and a reverse movement 
in the direction of the meridian perpendicular to it. In 
these cases, the astigmatism is to be recognized by the fact 
that when, near one point of reversal, the movement in one 
meridian has become indistinguishable, it can still be per- 
ceived in the other principal meridian. And, if the sur- 
geon places his eye at the point of reversal for the more 
myopic meridian and pushes the source of light a little 
away from the mirror, the erect movement in the meridian 
of less myopia, and absence of movement in the more 
myopic meridian become most distinct. It is upon this 
behavior of light in the pupil under these conditions that 



78 PRACTICAL APPLICATION WITH THE PLANE MIRROR. 

the diagnosis of the very low degrees of astigmatism must 
principally rest. 

The final test in any case will be made with the points 
of reversal brought together, usually at a distance of i 
metre or less. To do this, it will be necessary to place 
before the eye such a cylindrical lens as will correct the 
astigmatism, together with the spherical lens which will 
bring the point of reversal to the desired distance. With 
these lenses before the eye, the test is again applied. If 
the light in the pupil is found to move with the light on 
the face, the surgeon withdraws to a greater distance until 
that movement becomes indistinct. If the movement in 
the pupil is found to be against that of the light on the face, 
the surgeon approaches the patient until the movement be- 
comes indistinct. The apparent movement is to be care- 
fully inspected from the point of reversal and from a little 
within and a little beyond it. 

If it is found that the reversal occurs at the same dis- 
tance from the eye for all meridians, the cylinder chosen is 
known to be correct, both as to strength and as to the 
placing of its axis ; and the distance of this point of rever- 
sal from the eye indicates the amount of myopia which the 
spherical lens employed has caused, or has left uncorrected. 

If, however, the movement of light is found to cease 
in some meridian, but to continue (either direct or inverted) 
in a meridian at right angles thereto, it becomes evident 
that the cylinder chosen does not perfectly correct the 
astigmatism. If the astigmatism, thus found to remain, has 
the same principal meridians as those already fixed upon, 
the direction of the axis of the lens is correct, but its 
strength is not exactly right. Whether the strength needs 
to be increased or to be diminished will appear from the 
fact that the more myopic meridian continues to be the 
more myopic ; or that what was originally the less myopic 
meridian has become the more myopic. 



REGULAR ASTIGMATISM. 79 

If the astigmatism remaining after the cylindrical lens 
has been placed before the eye has principal meridians that 
do not correspond with those for which the lens is placed, 
the placing of the lens is incorrect, and the direction of its 
axis needs to be slightly varied, until the remaining astig- 
matism disappears or its direction corresponds with that of 
the lens before the eye. 

Where the cylindrical lens before the eye is of the 
right strength or is too weak, its axis needs to be turned 
slightly toward the axis of a similar cylinder which would 
correct the remaining astigmatism. If the cylindrical lens 
already before the eye is too strong, its axis needs to be 
turned toward the axis of a cylindrical lens of the opposite 
kind that would correct the astigmatism. 

The effect of such combinations of cylindrical lenses 
may be more fully understood by the study of the writer's 
paper upon " The Equivalence of Cylindrical and Sphero- 
cylindrical Lenses " in the Transactions of the American Oph- 
thalmological Society for 1886, page 268, or "Some Remarks 
on the Refractive Value of two Cylinders " by Carl Weiland 
Archives of Ophthalmology, 1893, P- 435> an( ^ J &94) page 28. 

When the meridians of any remaining astigmatism 
have thus been made to conform to the direction of the 
cylindrical lens before the eye, this remaining astigmatism 
has to be corrected by a change in the strength of the cylin- 
drical lens. 

For example : Suppose an eye having a compound 
hyperopic astigmatism corrected by -f 1. sph. 3 + I - C >' L 
axis 95 . The first inspection of the movement of light 
in the pupil shows a movement with that of the light on 
the face in all meridians, and a difference in the rate of 
movement in the different meridians so slight as prob- 
ably to escape notice. A convex 3 D. spherical lens will 
cause the movement in the pupil to be against the light 
on the face in all meridians when the eye is viewed from a 



80 PRACTICAL APPLICATION WITH THE PLANE MIRROR. 

greater distance than one metre. But it will also be noticed 
that the light moves more swiftly from side to side than it 
does upward and downward. 

If now the surgeon brings his eye closer to the patient, 
when the distance of one metre is reached, the movement 
of the light from side to side becomes indistinguishable, 
while there is still a very distinct movement against the 
light on the face upward and downward. . Approaching 
still closer, the movement from side to side is seen to be 
with the movement of the light on the face, the inverted 
movement still continuing in the vertical meridian. The 
movement horizontally with the light on the face, at first 
very rapid, grows slower as the patient's eye is approached, 
and the movement — against the light on the face — in the 
vertical meridian grows more rapid, until at a distance of 
one-half of a metre, the movement in the vertical meridian 
becomes indistinguishable, while there is a very clear move- 
ment of light, with the light on the face, from side to side. 

The point of reversal for the more myopic meridian 
(more myopic with the lens) has now been reached, and the 
surgeon keeping his eye at this position pushes the source 
of light away from the mirror. As he does so, the area of 
light in the pupil assumes more and more the appearance 
of a distinct vertical band, readily moved from side to side, 
but without apparent movement in the direction of its 
length. This band continues to become more distinct, until 
the original source of light is one-half metre from the mir- 
ror, and the immediate source consequently one-half metre 
back of the mirror, and one metre from the patient's eye 
— at the point of reversal for the less myopic meridian. In 
this position careful observation will show that the band 
of light in the pupil is not exactly vertical, but has the 
direction corresponding to the more myopic meridian of 
95 . The principal meridians then are located at 5 and 
at 95 . 



REGULAR ASTIGMATISM. 81 

Having determined this, the light is brought back as 
close to the mirror as possible, and the point of reversal for 
the 95 meridian is determined. To do this it may be 
advisable to change the convex spherical lens before the. 
eye, but whatever lens is employed, from the results ob- 
tained with it, the surgeon deduces the fact that in that 
meridian the refraction of the eye is hyperopic I. D. He 
then proceeds to measure in the same manner the refraction 
of the eye in the other principal meridian, finding with the 
convex 3 D. lens that this point of reversal is at one metre, 
and its refraction, therefore, hyperopic 2. D. The differ- 
ence between these meridians will be 1. D., the amount of 
astigmatism present. 

To make the final determination, there should be 
placed before the patient's eye the 1. D. convex cylinder 
with its axis at 95 ° and a 2. D convex spherical lens ; with 
which the point of reversal for all meridians will be found 
to lie one metre from the eye. If, in the placing of the 
cylinder, its axis is made to not correspond exactly with 
the meridian of least hyperopia, there will be found by 
this test a remaining astigmatism of low degree. Suppose, 
through carelessness or inaccuracy in the earlier observa- 
tion, the axis of the cylinder should be placed at 105 in- 
stead of at 95 °, the remaining astigmatism then would be 
found to be such as would be corrected by a convex cylin- 
der with its axis at about 70 . But on turning the cylin- 
der before the eye io° in that direction, that is, to its proper 
direction at 95 °, this remaining astigmatism would disap- 
pear. If, however, instead of the 1 D. cylindrical lens, a 
lens of 1.5 D. had been placed with its axis at 105 , there 
would remain an astigmatism which might be corrected by 
a concave cylinder with its axis at about 70 , and the turn- 
ing of the cylinder before the eye io° in that direction 
[to 95 ] would cause the remaining astigmatism to so 
change that its meridians would be at 5 and 95 , where a 



82 PRACTICAL APPLICATION WITH THE PLANE MlK±tOft. 

measurement of it would reveal the fact that the c\lmari- 
cal lens employed was 0.50 D. too strong. 

Aberration and Irregular Astigmatism. — The differ- 
ence in the refraction of different parts of the pupil is to be 
ascertained by measuring the refraction for each part sep- 
arately, just as though it were a case of simple hyperopia 
or myopia, care being taken to confine each observation 
strictly to the little portion of the pupil the refraction of 
which it is desired to ascertain. 

The amount of aberration or irregular astigmatism 
that is thus ascertained is of some scientific interest, and 
occasionally of practical importance as bearing en the 
prognosis of conical cornea, or of the changes of refraction 
in the lens which precede cataract. 

Generally, however, the important practical point 
about aberration or irregular astigmatism is its distribution. 
For practical purposes, the surgeon desires to ascertain 
which part of the pupil is free from any such defect, as 
that part will furnish the best visual zone ; and by what 
lenses that visual zone can be made most useful to the 
patient. The need for careful study to develop these points 
is sometimes great. Figures 22 and 23 represent the ap- 
pearances brought out by thorough investigation of a case 





Fig. 22. Fig. 23. 

of considerable astigmatism, coincident with equally pro- 
nounced positive aberration. Without careful study of the 
visual zone at the proper distances, it would have been easy 
to set the case down as one of aberration, and to have over 



ABERRATION AND IRREGULAR ASTIGMATISM. 83 

looked the regular astigmatism entirely. If the aberration 
encroaches decidedly upon the area of the pupil as deter- 
mined by a moderate light, it may be necessary to give a 
correcting lens, for use at near work and on exposure to 
bright light, different from the one required when the pupil 
will be somewhat larger. Or the surgeon may need to 
caution the patient that under certain conditions of light 
he must expect the correcting glasses to give slightly im- 
perfect vision. (See also page 68.) 

A fact to be borne constantly in mind in the applica- 
tion of skiascopy is, that it is not the high degrees of aber- 
ration and irregular astigmatism that are of most practical 
importance, requiring the surgeon to take into account 
their bad effects. More frequently it is the slight imper- 
fections of this kind situated within the portion of the 
pupil used for accurate vision that need to be recognized and 
taken into account, when prescribing glasses o± in giving 
an opinion as to the value of glasses. These low degrees 
of imperfection are to be recognized and studied only when 
the surgeon's eye is close to the point of reversal for the 
visual zone, after the effects of hyperopia, myopia and regu- 
lar astigmatism have been excluded by placing the proper 
glasses before the patient's eye. 

The investigation of aberration and irregular astigma- 
tism is the last step in skiascopy. A step very frequently 
not taken, yet essential to complete certainty and accuracy 
in the objective measurement of refraction. In a small pro- 
portion of cases, it will lead to modification of the glasses 
previously selected as best, and in a much larger proportion 
of cases it will discriminate sharply between the lenses 
which really best correct the ametropia and others which 
appear to give equally good, or almost equally good, sub- 
jective results. 

To carry it to completion will often require more time 
and effort than has been necessary for all other par L s of the 



84 PRACTICAL APPLICATION WITH THE PLANE MIRROI*. 

skiascopic examination. Nor is this strange, for it often 
includes the measurement of hyperopia or myopia, perhaps 
with astigmatism, in two or more areas of distinctly, though 
slightly, different refraction. It is not a distinct application 
of the test, but its application to separate parts of the pupil, 
instead of to the pupil as a whole. It requires no special 
directions and cannot be much elucidated by examples. It 
is to be mastered by a full understanding of the optical 
principles of the test, Chapter II, strict observance of the 
conditions of accuracy set forth in Chapter III, and the 
exercise of the needful care and patience. 

Measurement of Accommodation. — The objective de- 
termination of the nearest point for which the eye can be 
focused is possible only by skiascopy. It is sometimes of 
importance as in cases of suspected cycloplegia in children, 
or others for whom the subjective test cannot be relied on. 
In determining the condition of the accommodation in an 
eye with imperfect vision, or in recognizing any slight re- 
maining accommodation after the use of a mydriatic, the 
test is also of practical value. 

To make it, the surgeon first ascertains the refraction 
of the eye, and then places before it such a lens or lenses as 
will correct astigmatism and bring the point of reversal to 
a distance of one metre or a little less. He then places 
himself at this distance from the patient ; and directs the 
patient to fix his gaze upon some object on the farther side 
of the room, in such a position that the visual axis of the 
eye under examination shall pass as close as possible to the 
surgeon's eye. The point of a finger or pencil is then held 
close to the patient's eye, about the near limit of conver- 
gence and in the visual axis, so that the direction of the 
visual axis shall not be materially changed during the test. 

The patient is then directed to look first at the object 
across the room, then at the point of the pencil close to his 
eye. The surgeon by watching his other eye can ascertain 



MEASUREMENT OF ACCOMMODATION. 86 

whether the movements of convergence are really executed. 
Very strong convergence being impossible without strong 
accommodative effort, if any power of accommodation 
remains, the eye will be seen to grow more myopic when 
the pencil is looked at, and less myopic when the distant 
object is fixed, an inverted movement of the light in the 
pupil becoming apparent on " fixing " the near object and 
disappearing on " fixing " the other. 

To measure the amount of accommodation the sur- 
geon may approach the observed eye until the point of 
reversal is reached for the eye during very strong conver- 
gence ; or the lens before the eye may be modified in the 
direction of weaker convex or stronger concave until the 
point of reversal is brought, with a new lens and the 
accommodation, to the same distance as it was brought by 
the original lens without accommodation. The difference 
between the lenses in this case represents the amount of 
accommodation present. • 

For example : in a case of hyperopia 2 D. to ascertain 
if accommodation were present a convex 3 D. lens would be 
placed before the eye. This would bring to one metre the 
point of reversal of the eye with its accommodation relaxed. 
The surgeon at a little less than a metre could get through 
this glass an erect movement of the light in the visual zone 
when the patient was looking across the room. If now, on 
looking at the point of a finger held two inches in front of 
the eye, the movement becomes distinctly inverted, the 
light in the pupil moving against the light on the face, it is 
known that accommodation is present. In place of the 
3 D. lens a weaker convex may be substituted, and if the 
strong convergence of the visual axes still brings an in- 
verted movement of light in the pupil, a still weaker lens 
for this. In this way if it be found that with the 1 D. lens 
the patient is able by strong accommodative effort to bring 
the point of reversal to just one metre, the difference be- 



86 PRACTICAL APPLICATION WITH THE PLANE) MIRROR. 

tween the 3 D. lens and the 1. D.=2. D. will be the amount 
of accommodation present. 

Instead of changing the lens, the surgeon can approx- 
imately estimate the amount of accommodation by bringing 
his eye closer to that of the patient and finding the new 
position of the point of reversal caused by the exertion of 
the accommodation. Where much accommodation is pres- 
ent, such an approximate determination should first be made, 
but it is liable to the inaccuracies attendant on any skiascopic 
determination at a short distance. 

Mydriatics and Cycloplegics. — When in the dark room, 
the pupil fails to dilate to more than 4 mm. in diameter, 
skiascopy becomes difficult or impossible, and it is necessary 
to use a mydriatic. For this purpose a 4 per cent, solution 
of" cocain, a 2 per cent, solution of euphthalmin, or a solution 
of homatropine, 1 to 500, may be dropped in the eye. In 
persons who have unrelaxed accommodation, and this is 
sometimes true until fifty years of age or over, it is impos- 
sible to be certain of the amount of ametropia, without using 
a cyclop] egic. Either atropin, daturin, hyoscyamin, hyoscin, 
or scopolamin may be employed. But homatropine carefully 
used is equally reliable, and being more brief in its action, 
should be preferred. To secure complete cycloplegia, a 3 
per cent, solution of homatropin hydrobromate should be 
instilled every live minutes, for half an hour; and the 
measurements should be made one-half hour after the last 
instillation. 



CHAPTER VII. 

PRACTICAL APPLICATION WITH THE CONCAVE MIRROR. 

The Source of Light. — The room should be thoroughly 
darkened ; and to secure this, it is well to have the original 
source of light shaded. This light will, however, usually 
be back of the patient, and, except for the determination 
of the principal meridians of astigmatism, the farther it is 
behind the patient, the better. Hence, the shading of the 
light is not essential, as it is with the plane mirror. It is 
also of less importance that the original source of light 
should be small. Still the separation of light and shade 
should be as sharp as possible, so that the opaque shade 
with an opening opposite to the brightest part of the flame 
will be found serviceable. The opening in the shade may 
be two or three centimetres in diameter, so long as the orig- 
inal source of light is a metre or more away from the 
mirror. But when this is brought near the mirror to bring 
out the band of light in astigmatism the opening should be 
one centimetre in diameter or less. 

The surgeon places himself with his eye one metre from 
that of the patient. On throwing the light upon the pa- 
tient's face with the mirror, it is found that the area of 
light on the face moves with the mirror just as in the case 
of the plane mirror. The same reflections from the cornea 
and trial lenses are to be recognized and guarded against ; 
and within the pupil lies a similar portion of red fundus 
reflex, bounded by shadow, which is the subject of observa- 
tion during the test. If, however, the light in the pupil be 
seen to move with the light on the face, the eye is myopic 
more than i. D. If the light in the pupil be seen to move 

(87) 



88 APPLICATION WITH THE CONCAVE MIRROR. 

against the light on the face, the eye is hyperopic, emme- 
tropic or less than i. D. myopic. (See pages 23-26.) 

Hyperopia. — If the mirror be rotated about a vertical 
axis from right to left, the area of the light in the pupil 
will be seen to move from left to right, that is, against the 
mirror and against the light on the face. This is really an 
erect movement, we know from the demonstrations as to 
the real direction of the movement of the light on the retina 
in Chapter II. The difference between erect movement and 
movement with the light on the face must be borne in mind. 
With the concave mirror, the one is just the opposite of 
the other. The movement with the light on the face being 
an inverted movement ; and the movement in the pupil 
against the light on the face the erect movement. 

As with the plane mirror, the movement will be swift 
if the hyperopia be of low degree ; slower, if of higher 
degree. The convex lens is now to be placed before the 
eye, the swiftness of the movement in the pupil being the 
guide to the strength probably required. If the observer 
is not able to judge by this movement, let him at first 
employ in succession lenses that differ considerably in 
strength, as the 2, 4, and 6. D., increasing the strength as 
long as the movement in the pupil is against the movement 
on the face. 

When a lens is reached that causes movement of light 
in the pupil with the light on the face, slightly weaker 
lenses are to be tried until the two consecutive lenses are 
found, of which one gives the movement against the light 
on the face and the next stronger causes movement with 
the light on the face. Between these two lies the lens 
strength which would bring the point of reversal to the 
surgeon's eye. The surgeon's eye being one metre from the 
patient, this is the lens which would cause 1. D. of myopia ; 
and by subtracting 1. D. from its strength the hyperopia of 
the eye is obtained. 



MYOPIA. 89 

For example : Suppose hyperopia of 4. D. The light 
in the pupil will move against the light on the face at the 
first inspection, and also with the convex 2. D. and 4. D. 
lenses. With the convex 6. D. it is found to move with the 
light on the face. On trying the convex 5. D., the move- 
ment is indeterminable. With the 4.75 D., it is very rapid, 
but still against the light on the face. W T ith the 5.25. D., 
it is equally rapid, but with the light on the face. The lens 
strength between the two, or the 5. D., is then the one 
which causes 1. D of myopia ; and 5. D., the strength of 
lens, minus 1. D., the myopia caused by it, leaves 4. D., the 
lens strength required to correct the hyperopia present. 

Myopia. — In the mass of cases the inspection without 
a lens will show the movement of light in the pupil with 
the movement of light on the face, indicating the point of 
reversal is between the surgeon and the patient. When 
this is the case, concave lenses are to be tried, their strength 
being indicated by the rate of movement ; or, if this be nol 
a sufficient guide, they may be tried in series with an inter* 
val of about 2. D., until one is found which causes the 
light in the pupil to move against the light on the face. 

As the point of reversal is thus brought farther from 
the eye of the patient and nearer to the observer's eye, the 
light area in the pupil becomes more brilliant, and its 
movement more rapid. When the lens has been found 
which causes the light in the pupil to move against the 
light on the face, slightly weaker lenses are to be tried 
until it has been certainly ascertained which is the weakest 
lens that will cause the movement against that of the light 
on the face, and which is the strongest lens that still allows 
movement in the pupil with the light on the face. Be- 
tween these two lies the lens-strength which leaves the eye 
1. D. myopic. This lens-strength added to 1. D. will give 
the total myopia present. 

For example : Suppose the myopia present to be S.5 D. 



90 APPLICATION WITH THE CONCAVE MIRROR. 

The movement in the pupil without any lens will be very 
slow, and the light areas round and dim. Judging from 
this appearance the first lens tried may be the concave 5. D. 
With it, the light in the pupil will appear more brilliant and 
its movement will be more rapid, but it will still be ivith 
the movement of the light on the face. Next, the concave 
8. D. will be tried. The movement of light will be found 
still more rapid, but now against that of the light on the 
face. With the concave 7. D., it will be found equally 
rapid, but with the light on the face. With the 7.5. D. it 
will not be distinguishable. Hence, the 7.5. D. lens leaves 
1. D. of myopia still uncorrected, and this added to the 7.5 
D. corrected by the lens gives 8.5. D., the total myopia 
present. 

If the myopia be of low degree, the test without a lens 
will show either no distinguishable movement of light in 
the pupil [for myopia of 1. D.], or movement in the pupil 
against the movement of light on the face [for myopia of 
less than 1 D.] . In the former case the test is to be repeated 
with very weak convex and concave lenses [0.25. D. or 
0.50. D.]. The convex will give a movement of the light 
in the pupil with the light on the face, and the concave 
movement against the light on the face. 

If the movement is found to be against the light on 
the face to start with, the convex lenses are to be tried, 
commencing with a 1. D. lens, which, will cause the move- 
ment with the light on the face, and will show, therefore, 
that the refraction is myopia and not emmetropia, or low 
hyperopia. The weaker lenses are then to be tried, and the 
one which causes 1. D. of myopia thus ascertained. Since 
this lens is added to the myopia of the eye to cause 1. D. of 
myopia, it must be subtracted from 1. D. to find the amount 
of myopia originally in the eye ; the difference between it 
and 1. D. being the myopia present. 

Thus, in a case of myopia of 0.50. D. the light will be 



EMMETROPIA. 91 

found to move against the light on the face, without any 
lens, or with a 0.25. D. convex. But will be found to move 
with with the light on the face, with a convex 1. D., or 
0.75. D. ; and with an 0.50. D., the movement should be 
indistinguishable. The convex 0.50. D. then causes 1. D. 
of myopia, and subtracting it from 1. D. leaves 0.50. D., the 
degree of myopia previously existing in the eye. 

Emmetropia. — In emmetropia, on the first trial, the 
light in the pupil is found to move rapidly against the light 
on the face. With convex lenses it is found that the 0.75 D., 
or anything weaker, still allows this movement against the 
mirror. But the 1.25 D., or anything stronger, causes 
motion in the pupil with the light on the face ; and that the 
convex 1. D. causes no perceptible movement. Hence, the 
convex 1. D. lens causing 1. D. of myopia, the eye without 
a lens must be emmetropic. 

Regular Astigmatism. — The test beginning as for 
simple hyperopia or myopia, as the point of reversal for one 
of the principal meridians is brought near the observer's 
eye the movement of light becomes notably more rapid in 
one meridian than in the other, indicating the presence of 
this form of ametropia. When this is recognized, the lenses 
used are to be such as give a movement of light in the pupil 
with the light on the face in all meridians. Thus, if the 
eye has been hyperopic, the convex lenses used before the 
eye must be increased in strength until the movement with 
the light on the face occurs in all directions. Or, if the eye 
is myopic, the increase of strength in the concave lenses 
must stop so soon as any movement is seen in the pupil 
against the movement of light on the face. And the lens 
which causes this must be replaced by a weaker one that 
just allows movement with the light on the face in all 
meridians. 

The lens aimed at is the one which will bring the point 
of reversal for the least myopic meridian just to the sur- 



92 APPLICATION WITH THE CONCAVE MIRROR. 

geon's eye, one metre from the patient. If this is exactly 
attained, there will be in that meridian no perceptible move- 
ment of light and shadow, but the movement in the other 
principal meridian will still be with that of the light on 
the face. 

When this lens has been found, the original source of 
light, which, up to this time has been kept at as great a 
distance as possible from the mirror, is to be brought closer 
to the mirror, so that the image of it formed at the conju- 
gate focus in front of the mirror — the immediate source of 
light — will be removed farther from the mirror and closer 
to the patient's eye. 

The lens before the patient's eye brings the point of 
reversal for the least myopic meridian to the eye of the 
surgeon, and necessarily places the point of reversal for the 
more myopic meridian somewhere between the surgeon and 
patient. The object of bringing the original source of 
light nearer to the mirror is to carry the immediate source 
of light to the point of reversal for this more myopic merid- 
ian. As the light approaches its proper position, the area 
of light in the pupil becomes more and more band-like, 
being most distinctly so when the immediate source of light 
corresponds with the point of reversal for the more myopic 
meridian. 

When this is attained, the direction of the band is to 
be carefully noted as indicating the direction of the princi- 
pal meridian of least myopia. This direction having been 
determined and recorded, the original source of light is 
again moved as far away from the mirror as possible, and 
measurement of the refraction in the least myopic merid- 
ian completed as for a case of simple myopia or hyperopia. 

Then, the lenses are so changed as to bring the point 
of reversal for the more myopic meridian to the surgeon's 
eye, one metre distant from the patient ; and the lens that is 
found to do this, shows by the addition of I. D. if concave, 



REGULAR ASTIGMATISM. 93 

or the subtraction of i. D. from the strength if convex, the 
amount of myopia or hyperopia in the second meridian. 
The difference between the two meridians is the amount of 
astigmatism. 

When this amount of astigmatism has thus been ascer- 
tained, the cylindrical lens correcting it is to be placed 
before the eye and with it, the spherical lens, which will 
bring the point of reversal to a distance of one metre. 
The trial is then repeated, and if the point of reversal be 
found at the surgeon's eye for all meridians of the pupil, 
the determination already made is accurate. If, however, 
there be found distinct movement in the visual zone in 
some one direction while movement in the principal merid- 
ian perpendicular thereto is abolished, the cylinder selected 
does not perfectly correct the astigmatism. 

If this movement be in one of the principal meridians 
as previously determined [in the direction of the axis of the 
cylinder placed before the eye, or at right angles to that 
axis] the cylinder has been placed in the proper direction, 
but is too strong or too weak ; and its strength must be 
diminished or increased, according to the indications of 
the movement. If, however, the movement appears to be 
in a meridian different from either of the principal merid- 
ians as at first determined [different from the direction of 
the axis of the cylindrical lens before the eye, or the prin- 
cipal meridian at right angles to that axis] the axis has 
not been properly placed — does not conform exactly to the 
direction of the principal meridian. 

If this is the case and the cylindrical lens before the 
eye is of the right strength or too weak, its axis needs to 
be turned slightly toward the axis of a similar cylinder, 
which will correct the remaining astigmatism. If the cyl- 
indrical lens already before the eye is too strong, its axis 
needs to be turned toward the proper position for the axis 
of a cylindrical lens of the opposite kind that would correct 



94 APPLICATION WITH THE CONCAVE MIRROR. 

the astigmatism. Such a change in the direction of the 
axis of the cylinder is to be made, and the test repeated 
until the correction of any remaining astigmatism conforms 
exactly with the direction of the lens before the eye. This 
remaining astigmatism must be corrected by a change in 
the strength of the lenses employed. 

For example : Suppose an eye to have compound 
hyperopic astigmatism corrected by + 4. sph. 3 + 2 - C Y^ 
axis 90 . The first inspection of the pupil shows the light 
moving against the light on the face in all meridians. Con- 
vex lenses 2. D. and 4. D. placed before the eye show the 
same thing. Convex 6. D. shows the light moving against 
the light on the face from side to side, but with it in a verti- 
cal direction. It thus becomes evident that astigmatism is 
present. Still stronger convex lenses are to be tried. The 
8. D. lens shows movement in the pupil with the light on 
the face in all meridians. The 7. D. lens shows movement 
very indefinite or indistinguishable in the horizontal merid- 
ian, but clearly with the light on the face in the vertical 
meridian. This lens then brings the point of reversal for 
the less myopic [more hyperopic without the lens] merid- 
ian to the surgeon's eye. 

The next step is to bring the original source of light 
closer to the mirror, so as to cause the immediate source of 
light to fall at the point of reversal for the more myopic 
[less hyperopic] meridian, which will now be one-third of 
a metre from the patient's eye. To do this [supposing that 
the mirror has a focal distance of one-quarter of a metre, 
ten inches] it will be necessary to bring the source of light 
to within two-fifths of a metre of the mirror. That is, the 
immediate source of light to be at one-third of a metre from 
the patient, must be at two-thirds of a metre from the 
mirror corresponding with 1.5 D. of the focusing power. 
The total focusing power of the mirror being equal to 4. D., 
the light must be so placed that the divergence of its rays 



REGULAR ASTIGMATISM. 95 

will correspond to 4-1.5=2.5 D. That is, the light must 
be two-fifths of a metre from the mirror. When the light 
is in this position, the area of light in the pupil will assume 
the most distinct band-like appearance running in the di- 
rection of the principal meridian of least myopia [greatest 
hyperopia] , in this case horizontal. 

Having thus determined the direction of the principal 
meridians, one being known from the direction of the other, 
the original source of light is again placed back of the 
patient as far as possible, and the refraction in the horizon- 
tal meridian carefully tested by trying first the 6.5 D. spher- 
ical lens, and then the 7.5 D. spherical lens before the eye ; 
the former of which shows the movement in a horizontal 
meridian against the light on the face, and the latter a move- 
ment in the same meridian with the light on the face, thus 
fixing the refraction of that meridian as 7. D.-i. D. = 6. D. 
of hyperopia. 

Weaker convex lenses are then to be tried, until it is 
found that with the 5.5 D. the light moves with the light 
on the face in the vertical meridian, and with the 4.5 D. it 
moves against the light on the face in the vertical meridian, 
while the 5. D. gives no distinguishable movement in that 
meridian; showing that 5. D.-i. D.=4. D. is the amount 
of hyperopia in the less hyperopic meridian. The differ- 
ence between the two then is found to be 2. D., the amount 
of regular astigmatism present! 

The surgeon will then place before the patient convex 
5. D. spherical with convex 2. D., cylindrical, axis vertical. 
And on again trying the test, will find that he is at the point 
of reversal for all meridians. But, if on placing the cylindri- 
cal lens he make a slight error in the direction of its axis, 
placing it say at 5 one side from the vertical, he will find 
on testing the eye some appearance of astigmatism with its 
axis inclined several degrees in the other direction from the 
vertical. And, to get rid of this astigmatism, he has to 



96 APPLICATION WITH THE CONCAVE MIRROR. 

move the axis of the cylindrical lens to its proper position, 
pushing it toward the axis of a convex cylinder that would 
be required to correct this remaining astigmatism. 

If the case be one of slightly myopic, or high mixed 
astigmatism, the first inspection may show a movement 
with the light on the face in one direction, while the move- 
ment is ago hist the light on the face in the other meridian. 
This, of course, will indicate at once the presence of astig- 
matism. The fact, that it may occur, makes it important 
that the first observation on the pupillary movements should 
include the movements in different meridians. 

With the concave mirror [the immediate source of light 
necessarily lying as far in front of the mirror as its princi- 
pal focus, or even farther] if the astigmatism be of quite 
low degree, when the least myopic point of reversal is at 
the surgeon's eye, the more myopic point of reversal will 
be at the immediate source of light, or even closer to the 
mirror without any change in the position of the original 
source. Thus, the most distinct band-like appearance of the 
light in the pupil, the clearest difference between the move- 
ment against the light on the face in one meridian and the 
indefinite movement in the other merdian will be attained 
without bringing the original source of light any nearer to 
the mirror than its usual position. This must be born in 
mind for low degrees of astigmatism. 

Aberration and Irregular Astigmatism. — With the 
concave mirror, and the need of bringing the point of re- 
versal to a fixed distance from the patient's eye, the measure- 
ment of the amount of aberration and irregular astigmatism 
becomes very much more tedious and difficult than with the 
plane mirror, though not impossible. It is, however, not 
difficult to detect the presence of such defects ; and to as- 
certain which portion of the pupil they occupy, and which 
portions being comparatively free from them are available 
as a visual zone. As to the importance of such a study of 



MEASUREMENT OF ACCOMMODATION. 97 

the pupil, what has been said in the chapter on the plane 
mirror (page 83) will equally apply here. 

Measurement of Accommodation. — With the aid of 
lenses, usually concaves, the near point of accommodation 
may be brought to the required distance of one metre from 
the eye, and the amount of accommodation thus measured. 
The arrangement of the patient's and surgeon's eyes, and 
of the points to' be looked at, is the same as that described 
in connection with the measurement of accommodation with 
the plane mirror. It is, of course, impossible to make the 
approximate determination of the accommodation with the 
concave mirror by the surgeon approaching the eye of the 
patient. He must rely entirely on changes of lenses to 
bring the point of reversal to the fixed distance of one metre. 

Value of Skiascopy With the Concave Mirror. — On 
the whole the advantages of the plane mirror greatly out- 
weigh those of the concave ; but in certain respects the latter 
may be found superior. When the surgeon is without his 
usual facilities, and is compelled to make the examination 
with an unshaded light, the concave mirror is less handi- 
capped by this unfavorable condition. If, however, the 
electric instrument (page 104) be available, it will be more 
convenient and accurate than the concave mirror. 

In astigmatism, the concave mirror fixes with greater 
accuracy the meridian of least refraction, (axis for concave 
cylinder). In positive aberration, the retinal light area is 
most sharply outlined, when the immediate source of light 
is closer to< the eye than the point of reversal, as it is with 
the concave mirror. 



CHAPTER. VIII. 

GENERAL CONSIDERATIONS. 

Apparatus. — In the chapter of the Conditions of Ac- 
curacy, something has already been said as to the apparatus 
by which these conditions are best complied with. The 
two requirements to meet which the apparatus for skiascopy 
is to be adapted, are that it shall furnish the conditions 
necessary to the greatest accuracy, and that it shall facili- 
tate the finding of the lens that will bring the point of re- 
versal to the surgeon's eye. 

The Mirror. — As has been stated, the essential point 
in the mirror is the sight-hole small enough and free from 
reflections. This may be obtained by having the glass thin, 
if the sight hole is cut through it, having its margin free 
from chipping, beveled as little as possible, and thoroughly 
blackened with a dead black. 

If the sight hole is not cut through the glass, but is 
merely an aperture in the silvering, the glass may be much 
thicker and there is no ground glass to deal with. The 
difficulty with such a mirror is in keeping the exposed glass 
at the sight hole clean. Unless great care is taken in pre- 
serving it from dust, and carefully removing any that falls 
upon it, there will be a ring of dust in the periphery of the 
sight hole, which will irregularly reflect more light than 
would the ground glass edge of the perforated sight hole. 
And, it is difficult to keep this space entirely clean without 
chipping into the back of the mirror in such a way as to 
cause annoying reflections. But, however difficult, it is 
important to have the sight hole free from reflections. 

(98) 



THE MIRROR. 09 

The size of the mirror will depend somewhat upon the 
purpose for which skiascopy is to be used. If the mirror is 
to be employed to measure refraction of all kinds, to show 
the movement of light in the pupil with high uncorrected 
hyperopia or myopia, it must be large, to give the range of 
movement for the immediate source of light that is neces- 
sary to render evident the direction of movement in the 
pupil, when that movement is slow and the illumination of 
the area is comparatively feeble. 

The disadvantage of a large mirror is that it gives a 
large area of light on the face, especially when as with the 
plane mirror, the original source of light is brought close 
to it. And in this large area of light on the face only the 
light reflected by a small portion of the mirror immediately 
surrounding the sight hole is of any use when the point cr 
reversal is near to the surgeon's eve [see page 29 for dis- 
cussion of limits of the part of the retina visible in the 
pupil]. With a small mirror, making a small area of light 
on the face, it is easier to keep this upon the eve than it is 
to keep the similar limited portion of a large area properly 
directed. 

On this account, where skiascopy is used, after an ap- 
proximate estimate of the refraction has been made by the 
ophthalmoscope or other means, quite a small mirror is 
found convenient. By a large mirror is meant one from 
35 to 50 mm. in diameter. By a small mirror is meant one 
under 20 mm. in diameter. The mirror, or, at least, the 
opaque back that carries it, cannot be well reduced to less 
than 20 or 25 mm., because, if smaller than this, it will 
admit light to the eye from the original source, through 
the space around the mirror; and such light, though not so 
annoying as a reflection at the sight hole, is a serious hi 11- 
derance in the application of the test. The mirror plate 
then must be large enough to shade the eye. 

A large mirror having a metal cap with an aperture of 



100 



GENERAL CONSIDERATIONS. 



from 10 to 15 mm. in diameter, that can be slipped before 
the face of the mirror, or turned back at pleasure, will 
answer for all sorts of testing. Such a mirror 1 is shown in 
figure 24. As already indicated in Chapter III, the sight 
hole should be about 2 mm. in diameter. 




The handle of the mirror should be rather thick, so 
that a very slow even rotation can be secured ; for, as the 
point of reversal is approached, the magnified movement in 
the pupil becomes so rapid that only by moving the mirror 
more slowly, and making excursions of very slight extent, 
can this apparent motion in the pupil be readily followed. 
This difficulty of causing the immediate source of light to 
move slowly enough is diminished in proportion as the im- 
mediate source of light is brought closer to the mirror. 

The Shade. — The shade that covers the original source 
of light should extend far enough above and below the 
flame to prevent the escape of any considerable amount of 
light into the room. Where an argand burner is used as 
the source, a cylindrical shade should be 20 to 25 cm. long, 
with a diameter 6. or 6.5 cm., slightly greater than that of 
the chimney used, so as to allow a free current of air be- 
tween the shade and chimney and thus diminish the heat 
from the flame. An asbestos shade has been proposed by 
Dr. J. Thorington (Ann. of Ophthalmology and Otology, 1895, 
p. 5) on account of intercepting better the heat of the flame. 

1 Made at my suggestion by Wall & Ochs, of Philadelphia. Another form 
is described by Dr. James Thorington, Philadelphia Polyclinic, 1893, page 329. 



SUPPORT OF LENSES. 101 

The aperture of about 5 mm. for the plane mirror, 
or larger, for the concave mirror, should be opposite the 
brightest part of the flame, which ought to be broad enough 
to allow of slight change of position of the surgeon with 
reference to it, without its becoming hidden by the shade. 

The Lenses. — Ordinarily these are taken from the 
trial case and placed in a trial frame before the eye. It is 
important to have them clean and comparatively undam- 
aged by scratching. The trial frame should be such as to 
support the lenses well up before the eye and with their 
centres before the centres of the pupils. They must also 
be far enough away from the face to escape the touching of 
the lashes, and to prevent the condensation of moisture 
upon them. The interruption of the, red reflex from the 
pupil by such an occurrence prevents the satisfactory appli- 
cation of the test, and may be quite puzzling, because the 
reason for the obscuration is not immediately apparent; 
and it may be ascribed to opacities within the eye. 

Support of Lenses. — The trial frames have the ad- 
vantage over other supports for lenses to be presently men- 
tioned, that they keep a constant position with reference to 
the patient, so that a slight movement of the patient's head 
does not carry his eye away from the centre of the lens to 
its periphery or beyond. 

When the surgeon has learned to estimate by the rapid- 
ity of movement of the light in the pupil, the amount of 
ametropia remaining uncorrected, by following the plan 
here laid down of considerable intervals between the lenses 
until an approximation of the required lens has been made, 
the number of changes of lens for any case is not neces- 
sarily great. So that for any one who does not employ 
skiascopy on large numbers of patients daily, the trial frame 
and lenses will be found entirely satisfactory. 

Special series of lenses mounted in revolving disks 
have been arranged by Haines (Ophthalmic Review, 18S6, 



102 



GENERAL CONSIDERATIONS. 



p. 282), Burnett, Doyne, Couper, (Trans. Am. Ophthalmol 
Soc, 1888, p. 223), Wurdemann, and others, to 
save time by facilitating the changes to the 
lens required. Some of these have been de- 
signed for the patient to make the change of 
lens under the direction of the surgeon, and 
others to give the surgeon himself control of 
their movements. 

One of the simplest arrangements is that 
described by Wurdemann (American Journal 
of Ophthalmology, 1891, page 223), shown in 
figure 25. The lenses are inserted in a sheet 
of hard rubber which the patient holds by 
the handle, bringing before his eye the lens 
the surgeon may indicate. 

In an instrument suggested by the writer 
the disk is rotated by a rod one metre long and 
attached by a universal joint, so that it drops 
out of the way when not in use. 

The lens series runs from 7 concave to 7 
convex spherical, with 0.5 D. intervals, requir- 
ing to be supplemented by lenses in the trial 
frame, for high hyperopia and myopia, or 
astigmatism. 

An ingenious piece of apparatus having a complete 
series of lenses, both spherical and cylindrical, arranged for 
the purpose, is described by Lambert (Trans. Amer. Ophthal- 
mol. Soc, 1894, p. 196). It has the lenses arranged in two 
disks for the spherical lenses, and detachable slides for the 
cylinders, enabling the surgeon to reach the lenses wanted 
quickly. To be compelled to run over the lens series to 
find the one sought, would be a way of consuming time 
rather than saving it. Other forms of elaborate apparatus 
for the purpose have been suggestedby Sureau of Paris, 
and one by Perkins and Tait of Philadelphia. A series 



Fig. 25. 



MERIDIAN INDICATORS. IO3 

sufficient for the approximate testing of the majority of 
eyes may save time where many are to be tested, especially 
if the concave mirror be employed. The writer, using 
habitually the plane mirror, has discarded all special forms 
of apparatus, and depends on the trial frame and test lenses. 
Meridian Indicators. — In working with lenses in the 
graduated trial frame one may refer to its graduation to 
ascertain the direction of the bands of astigmatism. But 
in the darkened room this is not convenient. To meet 
this want, Thorington (Medical News, March 3, 1894) and 
Prince (Ophthalmic Review, July, 1894) have suggested 
disks specially graduated for the purpose; the former called 
an axonometer; the latter an inclinometer. Better than 
either of these is the disc shown in Figure 26 — 




Fig. 26. 

The wire stretched across, the suggestion of Mr. R. B. 
Finch, is made to conform with the direction of the band of 
light; and the graduation, visible in the dark room, gives 
approximately that direction without moving the frame from 
the face. 

A Distance Measure. — Where the concave mirror is 
employed, the distance remaining fixed throughout the test, 
it is only necessary that the surgeon should properly place 
himself at the beginning, and retain his position. He can 



104 GENERAL CONSIDERATIONS. 

then dismiss the consideration of the distance, or provide 
for it by the addition of I. D. to the concave spherical 
lens or the subtraction of i. D. from the convex spherical 
lens that brings the point of reversal to his eye. 

With the plane mirror no measure is necessary where 
the test is used only to approximate the refraction, the sur- 
geon soon learning to guess at the distance closely enough to 
be within 0.25. D. of the amount of myopia present with 
the lens fixed upon. But for exact measurement it is con- 
venient to have something to measure from the patient's 
eye to the surgeon's. This may be either a tape attached 
to the trial frame or lens disk (Burnett), and picked up and 
held to the surgeon's eye when the test is completed, or the 
ordinary metre stick. In either case, it is convenient to 
have the measure graduated in dioptric focal lengths, as 
described by the writer in the Medical News, June 27, 1885. 
Electric Light Skiascope. — An instrument using, as the 
source of light a small incandescent lamp, fixed in a tube 
attached to the mirror, was presented by H. Wolff at the 
meeting of the Heidelberg Ophtha.lmological Congress in 
1896; and a somewhat similar instrument has been made in 
this country by De Zeng. A strong convex lens is placed 
over the lamp in the tube; and by varying the distance 
between the lamp and lens, the rays may be rendered diver- 
gent, parallel, or convergent, giving' the effect of either the 
plane or the concave mirror, or of variations in the distance 
of the original source of light from the mirror (pp. 48, j6 
and 92). These lamps give approximately a point of light, 
a better source of light than is usually obtainable outside of 
the room especially fitted up for skiascopy. 

Other Instruments, combining a special source of light 
and series of lenses, have been devised. They may assist 
the beginner, but are not adaptable to all conditions, and do 
not favor the highest exactness. 



CHAPTER IX. 

EXACT SKIASCOPY. 

The procedures necessary to render skiascopy most 
exact are considered in a separate chapter, because they 
cannot well be carried out until the test is thoroughly under- 
stood, and somewhat familiar in practice. Then it is only 
when taken together that these procedures are of value. A 
small source of light cannot be used without a small sight- 
hole in the mirror. A small sight-hole in the mirror adds 
to exactness only as it allows the use of a small source of 
light. A short distance between surgeon and patient, de- 
mands accurate measurement of that distance. Accurate 
measurement of the distance is useless except when the 
distance is short. 

For most eyes exact skiascopy is practicable, only with 
the plane mirror. A concave mirror does not allow the 
immediate source of light to come close enough to> the point 
of reversal. (See Focusing of the Light on the Retina, 
p. $J.) Aberration and irregular astigmatism cannot be so 
well studied with the concave mirror (see p. 96) ; and it is 
largely by excluding the errors they cause that skiascopy is 
rendered exact. In the present chapter, therefore, skiascopy 
with the plane mirror is alone considered. 

The Dark Room. — Exact skiascopy is only possible in 
a dark room, and the darker the room the more easily is it 
practiced. Delicate and elusive movements of light and 
shade in the pupil, are not to be studied in a light room. 
any more than stars are to be seen by day. The contrast 
secured by the black screen around the source of light (see 

(105) 



IC6 EXACT SKIASCOPY. 

p. 36) is not enough. The observer's retina must be adapted 
to such work by the temporary exclusion of light. Looking 
directly at the source of light, will disable the eye for exact 
skiascopy, for some minutes. If the light needs adjustment, 
this should be effected without looking directly at it. Or it 
may be looked at with the other eye, the one employed for 
the test being screened from it. 

Source of Light. — In exact skiascopy the source of 
light must be brought close to the eyes of both patient and 
observer. It must, therefore, be adjustable as to height. 
Or, what is better, the light being fixed at the right height 
for the surgeon's eye, the patient's position should be ad- 
justed, so that his eye is brought to the same level. 

Working at such a short distance the horizontal move- 
ments of the light may not need to be so great as mentioned 
on page 69; but the patient should be so placed that the 
light can be brought within six inches of either eye. This 
bringing of the light so close makes it necessary to reduce 
the heat of the flame as much as possible. 

A single metal chimney surrounding the glass or mica 
chimney does much to moderate the heat. But it can be 
still farther diminished by using a double metal chimney, 
the two cylinders of which are separated by an air space of 
5 mm. or more. The inner of the cylinders may have a large 
opening, such as may be used for ophthalmoscopy. The 
outer cylinder must also have smaller openings, one of which 
will give the required source of light. These openings are 
placed at the same height, opposite the brightest part of the 
flame, so far apart that only one of them is before the large 
opening in the inner cylinder, at any one time, and the outer 
cylinder can revolve on the inner, so that any desired open- 
ing can be brought in front of the light. Such a chimney 
is made by Wall & Ochs of Philadelphia. The need to keep 
down the amount of heat increases the advantage of the 



SOURCE OF LIGHT. IO7 

Welsbach mantle, and acetylene flame, over the ordinary 
flames of gas or oil. 

The size for the opening for the original source of light 
has been discussed on p. 36. But the dimensions there rec- 
ommended are not those which will give the greatest exact- 
ness at a short distance. The diameter of the opening must 
be inversely proportioned to the skill and experience of the 
observer. But after sufficient practice an opening 2.5 mm. 
in diameter can be used with advantage. Such an opening 
can be employed with a mirror having a sight-hole 1.25 or 
1.5 mm. in diameter. 

The Mirror. — The essentials of a good mirror are de- 
scribed on p. 98. Growing familiarity with the movements 
of light and shade in the pupil and the movements of 
the mirror necessary to produce them, enables the surgeon 
to work with a smaller and smaller sight-hole, at the same 
time shortening his distance from the eye studied. Complete 
darkness in the room, and full adaptation of the retina to 
thorough darkness, also* aid greatly in this direction. Just 
how small the sight-hole might be made, it is impossible to 
say. One can see through an opening one-half or even one- 
quarter mm. in diameter. But the practical requirements in 
the direction of exactness can be met by using a sight-hole 
of 1.5 in diameter. With a sight-hole so small, it is espe- 
cially important that its space be entirely clear, and its edge 
smooth, and free from reflexes. Whenever the edge becomes 
ragged and uneven, the mirror should be re-silvered and a 
new sight-hole made. 

For exact skiascopy the mirror never needs to be more 
than one centimeter in diameter, and one of this size or 
smaller will be found most convenient. To execute slight 
movements accurately in the direction of the handle, as well 
as at right angles to it, the swinging-mirror skiascope sug- 
gested by the writer, may be substituted for the one shown 
on p. 100. 



108 EXACT SKIASCOPY. 

The Distance. — What has been said on this subject 
(p. 42) must be qualified by the statement, that for most 
eyes exact skiascopy is impossible at a distance of more than 
one-half meter. Certainty, accuracy, and ease in recognizing 
the form of the most important light area and its movements, 
require that the distance be short. It may need to be reduced 
to one-quarter meter or even less. 

For the accurate measurement of these short distances, 
it is convenient to use a tape measure, attached to the mirror, 
and graduated to quarter diopters of focal length. When 
the point of reversal has been determined, this measure is 
stretched, with the unoccupied hand, to the trial lens on 
the patient's face, and the distance noted 

Size of Pupil . — The accuracy of skiascopy depends 
on the approximation of the retinal light area, to a mathe- 
matical point or line (see pp. 30, 37 and 51 ). The reduction 
of the opening in the metal chimney, reducing the original 
source of light (p. 36) and the accurate focussing of the 
immediate source of light on the retina, by bringing it as 
close as possible to the point of reversal (p. 39) are simply 
to secure this close approximation. But such approximation 
is interfered with by aberration and irregular astigmatism 
(pp. 41 and 56). 

In most eyes these optical defects are present, chiefly 
in the periphery of the pupil, and their interference with the 
focussing of the retinal light area, can be prevented by con- 
tracting the pupil and shutting off this defective peripheral 
area. Even when such defects encroach upon the central or 
visual zone of the pupil, their effect can be reduced to the 
minimum, by reducing the pupillary aperture. 

Hence in many eyes, certain points can be better deter- 
mined by skiascopy with a small pupil. To determine these 
the test should be applied before the use of a mydriatic. If 
the mere darkening of the room, causes too much dilatation 



SIZE OF PUPIL. IO9 

of the pupil, as it often does in young eyes, the pupil can be 
contracted by having the patient accommodate for a com- 
paratively near point. Thus, if the observer is one-quarter 
metre from the patient, the focussing of the observed eye 
upon the upper edge of the mirror, will be attended by a very 
noticeable contraction of the pupil. Extreme contraction is 
not to be desired since skiascopy is very difficult, with the 
pupil less than four millimeters in diameter. 

The important points to determine by skiascopy, with- 
out a mydriatic, are the direction of the principal meridians, 
and the amount of astigmatism. Measurements so made 
may differ from those made with the eye under a cycloplegic, 




Fig. 27 

but often they do not. If they do, having both sets of data 
before him, the surgeon will be less likely to fall into error, 
than if he depended upon only one. 

When a cycloplegic has been used, or when the darken- 
ing of the room produces too great dilatation of the pupil, 
it is still possible, to diminish the influence of irregular as- 
tigmatism and aberration, by the use of the pupil stop or 
shield. A double stop of this kind is shown in Fig. 27. 

At each end is a disc, 10 mm. in diameter. Each disc 
contains a central opening, one 5 mm., the other 6 mm. in 
diameter. One of these is held in front of the pupil, so that 
the outer margin of the disc is just within the margin of the 
cornea. The bright point of corneal reflex may be taken as 
a guide to ensure that the opening exposes the central or 
visual zone of the pupil. Such a stop is not often necessary, 
but will occasionally be of great assistance. It should be 



IIO EXACT SKIASCOPY. 

held as close to the patient's eve as possible, the hand being 
steadied by the fingers against the patient's brow. Xo ar- 
rangement of the stop that does not keep it in the immediate 
control of the surgeon, will prove satisfactory. 

Fixing the Meridians of Astigmatism. — This is the 
first and most important point in the measurement of astig- 
matism by skiascopy. When the astigmatism is of suf- 
ficiently high degree to give a decided band of light (0.5 D. 
or upward) the direction of its principle meridians can be 
fixed most accurately by the method described on page j6, 
and the use of the wire disc, shown on p. 103. The prin- 
ciple meridians of lower degrees of astigmatism are better 
determined by use of a cylindrical lens, as indicated on p. 79. 

Sometimes, however, the direction of the meridians can 
be fixed more accurately by a modification of this method, — 
using a cylinder that increases the astigmatism, instead of 
correcting it. Thus, if the required correcting lens were 
convex 0.25 D. cylindrical with its axis at 90°, one might 
use instead, a concave 0.25 D. cyl., with its axis at 90°. This 
would give the appearance of 0.50 D. of astigmatism, caus- 
ing a distinct band, the direction of which could be ac- 
curately determined. 

If the direction of this band is that of the axis of the 
cylinder the cylinder has been so placed that its axis corre- 
sponds with one of the principal meridians. If, however, 
the direction of the band obtained, does not correspond with 
the axis of the cylinder, that axis has not been placed in the 
direction of the principal meridian. But the band will lie 
half way between the present position of the axis and the 
real position of the meridian sought. If when the axis was 
placed at uo°, the band should appear in the direction of 
ioo°, the real direction of the meridian would be 90°. 

Working From Both Sides.' — To make sure that an 
accurate determination of the refraction has been made by 



WORKING FROM BOTH SIDES. 1 1 1 

skiascopy, it is necessary not only to find the lens which 
seems to correct the refraction; but to go beyond this and 
demonstrate that any stronger lens would be an over-cor- 
rection. This is what is done as to spherical refraction by 
following the directions given on page 34, and in the mid- 
dle paragraph on page 71. It is done with reference to the 
strength of the cylinder, by repeating the test with cylinders 
stronger and weaker than the one supposed to correct the 
astigmatism. It is done with reference to the directions of 
the principal meridians, by shifting the lens until the band 
of light seems inclined in the other direction. In the case 
given in the above paragraph, the cylindrical axis should 
also be tried at 70°, and the position of the band of light 
thus caused (at 8o°) carefully noted. This plan of approach- 
ing the correction from both sides, is of value in all methods 
of measuring errors of refraction. But it is of especial im- 
portance in skiascopy. 

How To Reach Exactness — Exact skiascopy is only 
possible after much practice, with care to work out the cor- 
rect result in each case. But what is now known of it will 
enable the student to reach proficiency in much less time 
than has been required for the original development of the 
method. What has been said (Chapter I) upon the difficul- 
ties of the test and how to study it, should be carefully read 
and applied, until the importance of each hint, there given, 
is appreciated. The theory of the test, as set forth in Chap- 
ters II to V inclusive, must be thoroughly mastered. When 
it comes to exact measurements, all eyes present aberration 
and irregular astigmatism, which cause forms and move- 
ments of the ligfit areas, that it is impossible to describe and 
classify. These can only be understood by understanding 
the underlying theory of the test. 

Practice with the test must enable the surgeon to per- 
form certain movements, and automatically draw certain 



112 EXACT SKIASCOPY. 

inferences from the appearances seen, before he can fix his 
attention upon some of the details that are essential to ex- 
actness. Until such a mastery of the test is attained, the 
directions of the present chapter had better be ignored. In 
beginning it will be well to use a sight-hole 2.5 mm. in 
diameter. When the reflex is seen through this can be easily 
recognized close to the point of reversal, in most eyes it will 
be worth while to try working with a sight-hole 2 mm. in 
diameter. Only after that has come into easy habitual use 
is it wise to reduce the size to 1.5 mm. Each reduc- 
tion in the size of the sight-hole may be followed after a 
little time by a corresponding reduction, in the size of the 
source of light, with a shortening of the distance between 
the observed and the observing eyes, and its more accurate 
measurement. 

It should be observed that these steps become possible, 
not by the successive instructions of the teacher, but by the 
continued practice of the student. 

What Exactness May Be Attained. — In all eyes having 
sufficiently regular refraction to give them normal vision, 
it is possible to measure that refraction to within one-quar- 
ter diopter of lens strength, and to determine the meridians 
of regular astigmatism more accurately by skiascopy, than 
they can be determined by any other method. For a few 
eyes such exactness will be attained with difficulty, and may 
even require hours of careful study of the case. But for the 
great majority of eyes it can be readily reached in a few 
minutes. In many eyes it is not difficult to fix the lens 
strength within one-eighth of one diopter. For eyes that 
possess less than the normal vision, there is no method now 
known that approaches skiascopy in exactness. 

The exactness attainable by this method of estimating 
the refraction of the eye, is sufficient for all practical pur- 
poses. No other method is so worthy of being used alone, 



WHAT EXACTNESS MAY B£ ATTAINED. II3 

or having sole dependence placed upon it. Still to diminish 
the risk of error, which attends all measurements we make, 
the results of skiascopy should as far as practicable, be 
checked by independent methods, especially by the best 
forms of subjective tests. 



CHAPTER X. 

AUTO-SKIASCOPY. 

The surgeon can apply skiascopy to the measurement 
of his own refraction, by the use of an ordinary looking- 
glass, in addition to the apparatus commonly employed for 
the test. He simply practices skiascopy on the reflected im- 
age of his own eye. as he would on the eye of the patient. 
The glass in which his eye is reflected, will be spoken of as 
the "looking-glass' to distinguish it from the ''mirror.'" 
which is used as in ordinary skiascopy. 

One eye (the observing eye) is used to study the re- 
fraction of the other (the observed eye). Thus the right eye 
is used to test the left, and the left eye to test the right. 
Double the distance from the eyes to the "looking-glass,'' 
corresponds to the distance between surgeon and patient in 
ordinary skiascopy. If a variable distance is used, as with 
the plane mirror, the surgeon measures his distance from the 
"looking-glass" and doubles it. If he wishes to work at a 
fixed distance, as one metre, he simply places himself at half 
that distance from the "looking-glass.'' 

For the plane mirror the light, properly shaded, is 
brought close to the surgeon's eye, as for ordinary skias- 
copy. It is best placed on the side of the observing eye; and 
may well be so much to the side as to be shut oft from the 
observed eye by the bridge of the nose. For the concave 
mirror the source of light should be some distance behind 
the "looking-glass," sufficiently to the side of the observing 
eye to shine upon it, but not far enough to the side to shine 

114 



ARRANGEMENT OF MIRRORS AND LIGHT. 



115 



on the observed eye, which must be kept as much as pos- 
sible in the shadow. With the electric light skiascope the 
test is applied as with the plane mirror. 

First, when the test is resorted to with the plane mirror, 
the light-source close to it sends light to the mirror which 
the surgeon holds to the observing eye. From the mirror 
the light is reflected to< the "looking-glass," from which it 
is re-flected to the observed eye, and forms in it a light area 







Fig. 28— Course of rays in autoskiascopy. O Observing eye, P observed eye, 
R image of observed eye formed by reflection, M mirror, L looking glass, S source of 
light, T virtual image of source of light from which rays enter the observed eye. 
The dotted lines show the course of the rays from the source of light reflected by 
the mirror and looking-glass to enter the observed eye. The solid lines represent 
the course of the rays emerging from the observed eye, reflected from the looking- 
glass and passing through the hole in the mirror to enter the observing eye. 

on the retina. From this light area the light emerges, and 
striking the "looking-glass" is reflected to the observing eye, 
through the sight-hole in the mirror. The apparent move- 
ment of the light area within the pupil is, as it would be 
observed in the pupil of a patient, placed at the ap- 
parent position of the image formed by reflection behind 
the looking-glass. The movements of the light area are 
produced by the same movements of the mirror as in ordi- 
nary skiascopy; and the apparent movements in the pupil 
have the same direction and significance as in the pupil of 
the patient. 



Il6 AUTOSKIASCOPY. 

With the plane mirror it is easier to change the distance 
of the observing eye from the observed eye than in ordinary 
skiascopy, every inch of change in the position of the sur- 
geon making two inches difference in the distance from the 
observer to the reflected image. In moving the light away 
from the mirror to bring out more distinctly the band-like 
appearance showing the principal meridians of astigmatism, 
the distance that the light is moved is not duplicated by re- 
flection, but the interval between the light and the observing 
eye is simply added to twice the distance of the eye to the 
"looking-glass" to get the distance of the source of light 
from the observed eye. 

With the concave mirror, to get the source of light 
in the same relative position as when placed behind the 
patient's head, in the ordinary testing of a patient, it is 
necessary to place it farther behind the "looking-glass" than 
the apparent situation of the reflected image. It should be 
at least one metre or more behind the looking-glass, except 
when to> bring out most accurately the meridians of astig- 
matism it is brought closer to the mirror. With the concave 
mirror, as with the plane mirror, the direction and the sig- 
nificance of the movements of light and shadow in the pupil 
are the same as in ordinary skiascopy. 

Auto-skiascopy is from the first almost as easy as ap- 
plying the test to the eyes of another. The double part of 
observer and observed, each eye taking a different role, is 
at first somewhat puzzling, but when one has become accus- 
tomed to it, it rather facilitates the test. The observed eye, 
fixed on the "looking-glass," is conscious of the movement 
of the source of light reflected in the mirror in front of the 
observing eye. At first there is an inclination to fix upon 
this bright light. But, when one has learned to* overcome 
this inclination, the consciousness of the light may be 
utilized to help to bring the light area properly upon the 
observed eye. 



APPLICATION OF THE TEST. 117 

When watching by reflection the movements of light 
and shadow in one's own pupil, the fixation of the observing 
eye (and the observed eye also) is upon the reflected image 
of the pupil of the observed eye. The observed eye under 
these circumstances has its line of sight exactly perpendic- 
ular to the mirror, and were that part illuminated would be 
in position to see the reflection of its own fovea centralis. 
But the light is not reflected from the observed eye. It 
comes from the direction of the observing eye, and iJiere- 
fore, falls upon the retina of the observed eye to the tem- 
poral side of the fovea, illuminating this part of the retina, 
from which the light reflex returns to the observing eye. By 
auto-skiascopy, therefore, one measures not the refraction 
at the fovea, nor yet the refraction toward the disc, but the 
refraction of a point of the retina somewhat to the temporal 
side of the fovea. 

Probably the chief value of auto-skiascopy will be found 
in the opportunity it gives for practice with the test. For 
isolated students this advantage is important, for skiascopy 
requires a great deal of practice to develop its full possi- 
bilities. 



INDEX 



Aberration, 16, 36, 41, 43, 55, 59, 

63, 82, 96, 105 

Accommodation 84, 97, 104 

Accuracy 35, 39 

Advantages 5, 105 

Apparatus, 12, 35, 69, 98, 103, 106, 

109 
Apparent movement, 21, 22, 25, 

28, 29, 49, 53 
Appearances in pupil, 13, 30, 46, 

56 

Application 69, 86 

Area of light, 21, 23, 24, 30, 47, 53 

Artificial eye 17, 68 

Astigmatism, 7, 16, 36, 40, 45, 56, 

68, 74, 91, 96, 105, no 

Atropin 104 

Auto-skiascopy 114, 116 

Axonometer 103 

Band appearance, 7, 12, 31, 40, 46, 
48, 53, 76, 92, 96 

Batten 54 

Bowman 7, 10, 46 

Brightness of light 32, 36 

Burnett 102, 104 

Charnley 9 

Chibret 9, 10, n 

Concave mirror, 9, 23, 26, 39, 41, 
43, .5°- 53, 60, 62, 87, 97, 105, 116 

Conditions of accuracy 35 

Conical astigmatism 54 

Conical cornea 7, 64 

Contents '. . . .3 

Couper 8, 102 

Cuignet 8, 9, 10 

Cycloplegics 86 

Dark room 35, 105 

Daturin 104 

Derby 35 

De Zeng 104 

Difficulties n, 38, 70 

Dioptric scale 103 

Dioptroscopie n 

Direction. .46, 47, 53, 76, 91, 95 
Distance, 38, 42, 43, 44, 48, 51, 87, 
103, 106, 108 

Donders 7, 8 

Doyne 102 

Duboisin 104 

(1 



Egger . 11 

Electric light skiascope 104 

Emmetropia 23, 26, 74, 91 

Enlargement 29, 48 

Erect image 18, 25, 59, 88 

Exact skiascopy 105, in, 112 

Extra-visual zone 58 

Facial light area 23, 25 

Fantoscopie 10 

Finch 103 

Focal lengths 103 

Forbes 8 

Form of light area, 30, 46, 49, 55, 

59, 65, 66 
Fundus-reflex test n 

Galezowski 8, n 

General principles 18 

Haines 102 

Hartridge n 

History 7 

Homatropin 104 

How to study. . . .12, 49, 66, 68, 84 

Hyoscyamin 104 

Hyperopia 7, 22, 26, 70, 88 

Illumination. .. .21, 32, 35, 56, 60 
Illustrations, 19, 22, 24, 29, 31, 

47, 48, 54, 56, 60, 61, 65, 67, 

69, 82, 100, 102, 103 

Immediate source 21, ^7, 39 

Inclinometer 103 

Indicators 103 

Inverted image 18, 25, 88 

Irregular astigmatism, 7, 40, 42, 

43, 55, 82, 96 

Jackson, 9, 10, 32, 79, 102, 104 
Juler 9 

Keratoscopie 10 

Koroscopie n 

Lambert 102 

Landolt n 

Learning the test 13 

Lenses 101. 102, 104 

Light area, 21, 23, 25, 26, 30, 32. 

41, 49, 55, 59, 65, 67 
Light source. 21. 22. 35, 37. 60. 87, 

106 

19) 



120 



INDEX. 



Magnification of retina 29, 48 

Measures 103 

Mengin S 

Meridians. .45, 76, 79, 95, 103, no 

Mirror 35, 70, 98, 107 

Morton 9 

Movements of light, 18, 21, 23, 
24, 27, 28, 51, 53, &, 64, 67, 88 

Mydriatics 86 

Myopia 7, 18, 23, 26, 72, 89 

Name 10 

Near point 84 

Negative aberration, 42, 57, 62, 105 

Obliquity of lens 68 

Oliver 11 

Optical principles 18 

Original source of light, 21, 23, 38 

Parent 8, 9, 10, 11 

Pendulum movement 54 

Perkins 102 

Plane mirror, 9, 22, 26, 38, 39, 41, 

43, 50, 5i, 59, 62, 69, 105, 114, 115 
Point of reversal, 20, 33, 39, 45, 47 
Position .... 12, 39, 42, 48, 69, 87 
Positive aberration. .. .41, 59, 105 
Practical application. .. .13, 69, 87 

Preface 5 

Prince 103 

Principal meridians, 45, 76, 79, 95, 

103 

Pupil, size of 105, 108 

Pupil stop 109 

Pupillary shadows, 25, 30, 32, 46, 

55, 59, 66 
Pupilloscopie n 

Randall 42 

Rate of movement 27, 30, 64 



Real movement, 21, 22, 23, 24, 28 
Regular astigmatism, 7, 16, 36, 40, 

45, 74, 91, 96, 105 
Retinal area, 22, 23, 25, 27, 30, 

2,7, 41, 51 

Retinal enlargement 29, 48 

Retinophotoscopie n 

Retinoscopy 10 

Retinoskiascopie n 

Reversal 18, 33, 45, 47 

Scissors movement 66 

Scopolamin 104 

Shade 35, 69, 87, 100 

Shadows. .. .24, 30, 46, 55, 59, 66 

Shadow-test 10 

Sight-hole, 35, 37, 98, 105, 112 

Size of mirror 99 

Skiascopy ....n, 97, 105, in, 112 

Smith, Priestly 10 

Source of light, 21, 22, 35, 37, 69, 

87, 106 

Story 9 

Study of test 12, 49, 66, 68, 84 

Sureau 102 

Symmetrical aberration, 41, 43, 57, 

59, 61, 62 

Thorington 100, 103 

Tait 102 

Umbrascopy n 

Use of test 13, 15 

Visual zone 43, 58, 82 

Weiland 32, 79 

Wolff 104 

Wuerdemann 102 



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